Imaging the CX in Pregnancy: Helpful Hints - HD

Introduction

My name is Faye Lang.
I'm a radiologist at Georgetown University Medical Center in Washington DC,
and today I am gonna be discussing the use of ultrasound of the grave cervix.
Today we're gonna discuss imaging the cervix in pregnancy using ultrasound
and hopefully address some issues that we can call some helpful hints.

Measuring the Cervix

When we think about measuring the cervix,
we typically begin at about 24 weeks
and we can see there's a range here from the 10th to the 75th percentile.
And the normal range is somewhere between 26 and 50 millimeters.
The reason we choose 24 weeks is because that is more or less the cutoff at which a baby can be born
after 24 weeks and survive, whereas before that time most of these babies will not be viable outside the uterus.

Visualizing the Cervix

When we look at the cervix, as we can see here,
diagrammatically from Dr. Netter's diagram, the length of the cervix seen
between this line and this line measured that way.
There are several ways to measure the length of the cervix.
Certainly the most common is the digital pelvic exam,
but there are problems with using this.
It's the most common, but it has anatomic problems.
One is, as we can see here, the upper border of the vagina ends at this point here where the fores are.
So if you're doing a digital exam,
you certainly can't palpate the proximal half of the cervix.
It's a subjective evaluation.
If you do a digital exam, you say, I think it measures so many centimeters long,
but you can't take a picture of that.
There certainly could be a risk of infection,
especially if the membranes are ruptured
and even if they're not, if the membranes are coming down through the cervix, you could potentially rupture membranes.
So it has a problem if you compare the digital measurement of the cervix compared to ultrasound
using vaginal ultrasound, we can see that on ultrasound the length of the cervix is considerably longer than the digital exam.
But that should not surprise you because again, when you measure the cervix digitally,
you're missing the whole proximal half of the cervix.
So ultrasound is really the gold standard for evaluating the graver cervix.
There are several ways this can be done, transabdominally, translab, and transvaginally,
and we'll talk about each of these, starting with the transabdominal examination to measure the cervix,
the most common way to do it, but the least accurate.

Transabdominal Approach: Problems and Solutions

What are the problems with using a transabdominal approach?
Some of it is a problem with the bladder.
It could be too full.
It could be too empty,
and sometimes either due to an empty bladder or overlying fetal parts, the cervix may be obscured.
So here's an example where the bladder is too full.
It compresses not just the cervix together,
but it drags the lower uterine segment, the anterior wall of it against the posterior wall, causing it to look too long.
It increases the measured length of the cervix.
And actually the cervix is not that long.
It's just compressed in the lower uterine segment together.
If it does so, as in this case in the placenta,
as we can see here, gets dragged along with the lower uterine segment,
you could think there might be a placenta previa.
So that would increase the false positive diagnosis of placenta previa V.

How do you address this really from a practical point of view?
Here's a lady who gets an A plus for filling her bladder.
It's very distended and it doesn't really look like there's any problem.
I would very much urge you to realize that ultrasound is not done in a vacuum.
You need to have a clinical history.
And in this woman, actually she had recurrent miscarriages in the mid portions mid trimester of her pregnancy.
So this is not an acceptable view because we already said her bladder might be compressing the cervix.
So in this woman, when we emptied her bladder and went on to the next way of looking at her,
in this case a trans labial scan, we can see that her cervix is entirely dilated.
It was just being compressed by this full bladder and between the calipers here are the, is the endo cervical canal.
So she's completely dilated and that was completely missed due to the fact that her bladder was so distended
and compressed the walls of her cervix.
But it was her history that was the clue not to allow us to stop the scan at this point when her bladder was so full.

Having an empty bladder can also be a problem if it's too empty, there can be acoustic shadowing as we see here,
denoted by the asterisk and there is loss of the acoustic window.
As we can see, we just don't have a good window to work through.
Now why are you seeing the shadow here?
One thing is you may be scanning over the pubic bone and you may be seeing shadowing from the bone.
Another is with the empty bladder.
There's an angulation problem here between the bladder and the fluid and the uterus causing some of the sound
to be reflected away from the transducer and hence the acoustic shadow.
So these are technical problems that can lead to problems.
A completely empty bladder, as in this case may be confusing.
Somebody who doesn't have a lot of experience may look at this case and think, wow, that cervix looks entirely dilated
with this being the internal and not the external cervical us.
And in point of fact, whenever there's a problem where you're questioning something like this, think it's a good idea
to scan the patient in a different manner.
In this case, we went on to a trans labial scan.
We can see her bladders complete.
Her cervix is completely closed, the anterior versus the posterior wall,
the endo cervical canal here completely closed.
What happened here was when her bladder was totally empty,
as we can see here, something has to occupy the space where the bladder was.
And so what happens is her uterus actually becomes more vertical and so does the cervix.
So here would be the internal O, the external os,
the endo cervical canal is seen.
It's a line in this position and you're only, you really can't resolve it
because you are looking straight down along its course coming in by a trans labial or if you wanted a transvaginal route.
This way we're 90 degrees to the position of the cervix and therefore now here's the coming down
with the trans labial scan.
We're pointing directly looking at the endo cervical canal and hence we can resolve this issue.
Sometimes again, acoustic shadowing is seen here, limiting our ability to look at the cervix.
And again, the bladder's quite empty.
Is there a better way we can approach this cervix?
And the answer here is yes, and we see not an ideal picture perhaps,
but certainly I can measure the length of the cervix here.
Internal external os.
What is the difference between these two scans?
The difference is the transducer that I chose on the first image, I'm using a large faced curved array transducer,
and here I am using a sector scan, a sector probe.
I can position better in areas of limited view.
I can push against the baby's head, which we did here.
Notice here the head is lower by taking that probe and coming just over the pubic bone, pushing a little bit with it,
getting the presenting cranium out of the way, I now have a better ability to look at the cervix.
So I would like to make a pitch if you can.
If you have this transducer available, a sector in cases where you can't see the cervix well
and you're scanning transabdominally, change to a smaller face transducer such as the sector probe.
See if you can manipulate things and get a better look at the cervix.

What about this case?
Again, using a sector scanner, abdominally, it looks like the internal US is open
and people might say there's a dilated cervix at that point.
But again, if there's a question and you're unclear, look at it from a different approach.
Here again, we went to a trans labial scan and we can see that the cervix from here to here is measuring 34 millimeters and it's normal.
So what is going on in this case?
Why is the cervix normal here? But what is the problem here?
People who have had experience quickly recognize that what we're dealing with here is a lower uterine segment contraction,
either a contraction that may be resolving or one that's actually forming and it has this typical keyhole appearance.
If you could force yourself to look beyond that keyhole here is the normal cervix,
but I urge you if you have a question, go ahead and look at the patient using an alternate approach.
And by that I mean either a trans labial or a transvaginal approach.

So we've already mentioned that there are problems with transabdominal scans, but we usually get away with it.
It's the most common way we look, but we've discussed some of the problems
as you're looking at the cervix transabdominally, I also would suggest you take dedicated images of it,
do some cone down views of the cervix.
Look more than once, particularly if a woman is having contractions because when a woman has a contraction,
the cervix tends to shorten and dilate and that would be a time when you might see that it's not looking quite as normal
as it did when she was not having a contraction.
Have the bladder relatively empty, not too full where you can compress the cervical walls together
and not too empty, sort of moderate and very important in yellow italics.
Patient's history is critically important as to when you might wanna go and do either a trans labial or a transvaginal scan.

Translabial Scans

So let's next address trans labial scans and what we're doing here, schematically,
you can see the transducer, you can see it has in this case we're using a sector
and you can see the orientation of the cervix.
Notice the patient's bladder would be empty when we do this.
And what's really important for those of you who have not done trans labial scans
and the same is addressed with transvaginal scans, is to get used to how you look at the monitor,
which is 90 degrees counterclockwise.
You always put the top of the probe at the top of the image on the monitor,
even though you can see here we're coming in from the patient's feet toward her head.
What we do is we take the transducer and then it depends on, you can cover it with some kind of protective device.
And one of the things we use is just a plastic sandwich baggie.
You put a little transducer gel over the probe, then you cover it with a baggie, a sandwich baggy,
and then you put a little more transducer gel and you're ready to go.
And hopefully you can image and see what looks like the cervix, just the landmarks that we're looking at in.
Now we said the transducer probe is here, so it's looks like the patient's feet are pointing to the ceiling.
That's obviously not the case, but the image on the monitor is rotated 90 degrees counterclockwise.
So that's where the vagina is.
This is the anterior and the posterior cervix that we're looking at.
And we can see the endo cervical canal right there.
And the problem with this approach you can see here is acoustic shadowing due to overlying bowel loops.
The rectal sigmoid typically in this area making it very difficult, if not impossible in a fair number of cases
to see the external cervical OST due to that shadowing.
In which case, if you are being asked to measure the length of the cervix and there's bowel in the way,
you won't be able to get an effective measurement.
If you are new at this and you're having trouble lining up the anatomy as I just showed you,
I suggest you start with a coronal scan, which is basically holding the probe horizontal, or transversely
as you place the probe at the level of the introitus and start with the coronal scan.
And what it looks like is similar to what you might see if you were placing a speculum in and looking for the cervix.
You can see the rounded cervix.
This is the endo cervical mucosa.
The little horizontal line there is where the walls of the cervical walls come together
and that is the endo cervical canal itself.
And take that line and then pivot on it.
And when you pivot on it, hopefully you'll see that endo cervical line as we're seeing here, endo,
cervical mucosa and the anterior and posterior walls of the cervix.
So hopefully that little tip will help you in your orientation.

When to Use Translabial or Transvaginal Scans

Now when do you do these trans labial transvaginal scans?
Well, we've already said when the transabdominal scan, if you're having a question
and why do you usually have a question?
Will, you're often worried Is there a problem with the cervix?
May it be dilated or efface?
If there's a question of placenta previa and you're not seeing the relationship of the placenta to the internal loss,
well on a transabdominal approach, you go to the next level.
So when might the cervix be a faced or dilated?
Any woman who has a history in the past of preterm labor is at risk for having it again.
And I did show you one example where it looked okay with a very distended bladder
and it was totally dilated when we went to the trans labial scan.
So if there's a question of cervical incompetence, is her cervix not holding up well?
If she has preterm rupture of her membranes, any patient where there's a question of ruptured membranes,
you do not wanna do a vaginal scan, because the potential is you could introduce infection if the membranes are ruptured.
So again, a trans labial scan would be a good way to look at that woman following placement of a cerclage
is another situation where you might see it better using a trans labial scan or a transvaginal scan.

Cervix Dilation Patterns

So let's look at this diagram.
What happens to the cervix when it dilates?
And these diagrams tell you what happens to the cervix, whether you're dilating at term or preterm.
The non dilated cervix, we're seeing the anterior and the posterior walls of the cervix here schematically
the endo cervical canal here and its relationship to the internal loss forms the letter T.
Now as the cervix begins to dilate either at term or at preterm, it then dilates from the internal loss
toward the external loss.
So here diagrammatically, it assumes the letter YV and then ultimately as it's entirely dilated, the letter U
and Dr ante in this article a number of years ago, this was from his work
and he says, trust your vaginal ultrasound TYVU.
And I just thought that was kind of cute.
You don't really have to remember it, but just remember that the internal OS dilates first
and then the dilatation progresses from the internal down toward the external os.
So here we have a nice example of the letter T, the cervix is completely flattened at the internal OS level here
and that's what we'd expect.
And I'm gonna draw your attention to the fact that if you really wanna see that internal os,
it's important to get a little bit of fluid if you can, to be just in front of the internal os.
So you have the conspicuity to see it, you've got to try therefore to pull the presenting part up away from that internal loss.
One of the things I do and remember, these women have empty bladders, so the baby wants to come down low
is ask the mother if she could to place her hands low in her lower pelvis, see if she can actually pull the baby up.
It only takes a couple of millimeters to give you fluid and get a scan that is technically optimal.
And I'm gonna come back to this point of why it's so important to have a little bit of fluid in front of the os.
One is, so you can see the internal OS and I'll get back to this point later as to another reason why it's important.
So here's a normal cervix, the letter T, here's one that's starting to dilate again from the internal OS letter Y,
more dilated the letter V.
And this one is completely dilated the letter u.
Now notice that this scan, this is actually done transabdominally, we start all of our scans initially transabdominally,
if I saw what looked like a completely dilated cervix as in this case, I don't really think there's any indication
to do a transvaginal scan.
You might wanna do a trans labial scan but not a transvaginal scan.
Again, you could have the membranes starting to herniate through.
You do not wanna possibly rupture the membrane and your probe itself might cause the already dilated cervix
to become even more irritable.
So if you see a completely dilated cervix like this, it's time to get the clinician involved quickly.
Don't stand the patient up, just get her upstairs on a flat gurney.
You don't even want the pressure when she stands of that fluid to push down against her cervix
because in that process perhaps the baby might start be coming out as well.
So that's, you know, once you see a dilating cervix, get the patient to have some, you know, see the obstetrician
and don't have her stand up.

Measuring Cervical Length

Now when we measuring the cervix, the most important measurement is to measure the residual length of closed cervix
as the cervix dilates as we can see here from the internal loss, the lower aspect of the lower uterine segment
and the upper cervix sort of become one unit and it's really impossible to measure I think where the cervix actually started.
At what point is this? The internal O?
So we wanna concentrate on the, the area of the cervix that is not dilated.
So you've gotta see this point and then it's incumbent upon us to also see the external OS as you can see here.
So that's what you wanna measure.
And what measurement do we start getting worried about?
Well the magic number is 30 millimeters.
Remember I said the normal cervix has a range and I said it starts at 26 millimeters.
But from a practical point of view, 30 millimeters is considered a good cutoff.
We will have some false positives.
There will be some women whose cervix is less than 30 millimeters, but she's not abnormally dilated.
But the reason this is an important number, you can see Dr. ETA's considerable work in public publications on the cervix.
And in this particular very good article, with a discussion about the probability of preterm delivery.
If you take 30 millimeters now this is 20 zero, so we're shortening our cervix
and the probability of delivery you can see starts to rise very dramatically as the cervix progresses
to become shorter and shorter.
So it's our philosophy that once the cervix measures 30 or less, we get on the phone and we call the clinician.
And you can see a comparison here between ultrasound and the clinical evaluation vis-a-vis predicting preterm death,
or I'm sorry, preterm birth before 36 weeks.
In terms of these importance, statistics ultrasound does better than the clinical exam with respect to evaluating that cervix.
So it's really quite critical to be able to measure that cervix.
And again, 30 millimeters or less is concerning.

Hourglass Cervix

Now sometimes the cervix has an unusual shape known as an hourglass cervix.
What happens here is that the cervix is completely distended, but it's not all that dilated
because fluid herniates out where the membranes herniate through the inter the external O into the vagina.
So now the vagina contains membranes herniated through with fluid and it has a greater ability to distend than does the cervix.
So the cervix may be rather non dilated compared to the fluid in the vagina.
Hence you get this more or less hourglass configuration.
And to those of you who have not seen it before, it might be confusing because you may fail to understand the anatomy.
So let's look at this hourglass here.
Hourglass scene on this little diagram here or picture, what are we looking at?
That is the cervix.
And you can see it is not as distended as the fluid that's in the vagina.
And when you see this, notice again we're starting transabdominally.
This is a patient in whom you do not wanna put anything into her vagina because you might rupture those membranes
or you may give her an infection and that is a disaster.
You get on the phone, you call the clinician, you don't let the patient stand up,
you send her upstairs on a gurney to be evaluated by the obstetricians at this point.
Here's another example.
And again, notice how this endo cervical canal, which is completely dilated, is not that distended.
The diameter is not that great because so much fluid is present in the vagina, which can accommodate this large amount of fluid.
Notice also in this case what's coming into the endocervical canal is the umbilical cord.
So that's a really bad situation when you have a cord presentation.
And here's another one, not that dilated but accommodating a lower extremity and the foot which is sitting in the vagina.
And here are the membranes again that you can see herniating into the vagina containing echogenic fluid.
That's not uncommon late in pregnancy.
So several examples now to recognize of hourglass cervix and all done and appreciated with a transabdominal scan
and you do not want to do a transvaginal scan in these patients.

Pitfalls in Cervical Evaluation

So now that we've discussed varying approaches to look at the cervix, some of the problems we've mentioned the hourglass cervix,
let's talk about some of the problems you may encounter.
Here's the patient measured on a transvaginal scan, EV transvaginal.
And the cervical length looks fine, it's about four centimeters in length, but now it's two and a half centimeters.
What's going on here?
That's too short but that's normal.
What is the difference between these two scans?
The difference is that this woman by history, it sounded like she had an irritable cervix.
So what we actually did here was we put pressure with our hands on the upper aspect of her uterus toward the fundus.
And with our hands we pushed from the fundus in a direction down toward the cervix.
We were putting pressure against her internal loss.
And because it was irritable, her cervix started to dilate again by the internal loss first.
So it decreased by 15 millimeters with that IC pressure.
That's analogous to if a woman, again, if she's having a contraction, then physiologically she's putting pressure against that internal loss,
which is why, as I've already said at the time of a contraction, go and look at the cervix because it might be getting shorter
if she's not actively having contractions.
But you're concerned, see if you can put pressure against it by putting pressure on the fundus
and pushing down in the direction of the internal cervical os.
So here's an example where that cervix looks relatively, okay, now I'm gonna show you a clip
as we're applying fundal pressure in this woman as we do so, you can see how her internal OS is starting to dilate
and her cervix is getting shorter.

What about this patient? This cervix looks fine.
It's more than four centimeters long. Looks great.
But now within a period of just a couple of minutes or so, her cervix has shortened greatly
and what's going on here in two minutes it spontaneously decreased.
She was having a contraction, she had an irritable cervix.
So be aware of this.
One other point I'd like to make when you put the vaginal probe to do this,
don't just put it in and then just pull it out 10 seconds later because it's just my general feeling
that when you first put that probe in the cervix, if it was dilated may decide it doesn't like the probe in there
and it starts to go back to a more normal appearance.
So when you put the probe in, I would suggest you keep it for at least one or two minutes, talk to the patient,
try to get her to relax and see if you have a more physiologic appearance to that cervix.
Because when you first put the probe in, I think the door is slammed shut.
And that is something you don't wanna have a normal looking cervix if it's really dilated.
So you wanna pick up all the cases you can.
Here's another example where you might think that cervix looks.
It's a little bit of maybe the letter Y might come to your thought here because of that V like appearance at the internal loss.
But look at this cervix. Oh wow, look at it.
It's totally dilated. Well why is it here?
It looks okay and here it's so totally dilated, it doesn't look absolutely okay but certainly a lot worse here.
What's happened here is that when that vaginal probe was put in, it was compressing the walls of the cervix.
In other words, we were pushing with the probe compressing the anterior against the posterior wall of the cervix.
I'm gonna show you a clip in just a second. Here we go.
As I push with the vaginal probe, I can push the anterior wall of the cervix against the posterior wall.
And I would suggest you not do this because I think if the cervix is already this irritable,
it's not gonna be happy when you push the probe like this.
I also wanna mention I'm not pushing the probe over and over and over here.
This is just one push.
But the clip goes around and around and around.
Once I see that the cervix is dilated, do not push that probe in so hard that you're compressing its walls.
You don't wanna have a normal looking scan when it's not normal.
Here's another example.
That cervix looks perfectly fine and if you measured it, it measures, it's a normal measurement.
Notice also where we put our calipers 'cause it's a slightly curved cervix.
So this is a sum of these different measurements.
This one plus this one is a normal length cervix, but now you measure the same patient,
it's only two centimeters and this is again an effect of pushing too hard against the cervix with the vaginal probe.
And I'm gonna show you a clip in this case to show you what's going on.
Cervix is too short here and the clip now as I'm releasing pressure against the cervix with the probe,
I think you can appreciate it was longer and now it's shorter, it's longer and now it's shorter as I'm pulling back with the transducer.
So don't push that cervix, don't make it look closed or longer than it really is by having transducer pressure against the cervix.

What about this case? This is done transabdominally.
Now if you're busy and somebody just shows you this, it might be very easy to say that looks like a normal cervix.
Oh, but now it doesn't.
Now it looks like there's fluid in the end of cervical canal.
So what's going on here?
Actually that's not the cervical wall that is a foot in the endo cervical canal.
Here it is. You see it sort of moving just a little bit and then this baby cooperated pulled the foot right out of the end of cervical canal
and you can see it's a totally dilated cervix.
So there are some situations which I'll call a false negative because the cervix looks normal but it's really not.
And you wanna avoid these situations.
The things again to review that could get you in trouble is that overly distended urinary bladder.
If the cervix is irritable but the patient's not actively contracting and you fail to do fund pressure maneuver
that is pushing your hands, taking them on the top of the fundus and pushing them down in the direction of the internal loss.
Intermittent dilatation, pushing too hard with the probe, compressing the walls of the cervix,
and an unusual situation where there may be fetal parts in the end of cervical canal.

Now let's flip the page and see if we, the opposite might occur where you might think the cervix is abnormal
in terms of fluid in it or distension and it's really not okay, a false positive diagnosis.
Let's look at some examples.
This cervix looks like it might be dilated 'cause it looks like there could be fluid in the end, endo cervical canal.
But in point of fact, if you look at the clip carefully, that's a little white line here that you're seeing,
which is actually the opposing walls of the cervix.
That's actually the endo cervical canal.
And so therefore there's not fluid, but most people have more echogenic mucosa of their cervix.
So in this case, for whatever reason, I don't know that endo, the echogenic endo, cervical mucosa is lacking.
It's unusually loosened.
But if you look at this case Sally and coronal, there's, there's no line here, there's no.here.
That is actually fluid in the endocervical canal.
Lack of seeing that endocervical stripe.
And I'd suggest you do this in two directions and look completely from side to side.
Make sure there's no endo, cervical dot or stripe here to suggest that it's loosened, mucosa.
So that's truly dilated.
And this is pseudo dilated because you're seeing that endocervical line here.
So that's very, very important.
You don't wanna miss a case and you don't wanna over call a case.

What about this case?
Well the cervix is, is trans labial measures 27 millimeters, that's shorter than the 30 millimeters.
So we shouldn't be happy with this measurement.
But is this really a good scan? Again, it's trans labial.
Could there be a technical problem?
Well look at the big time shadowing here.
Obscuring the external loss.
That's due to the rectosigmoid bowel gas.
When you have that, you can't have a tech, it's a technically suboptimal scan
and you know, we're always wanting to put calipers on and do measurements.
Well be careful before you do that, make sure your scan is technically appropriate
because that bowel gas is causing a problem here.
Her cervix is not short.
When we did a transvaginal scan in this case you work around that bowel gas and now we have a normal length cervix.
Okay? So that would be, that's an important false positive.
That's a major problem with a trans labial scan.
The inability to see the external loss because of acoustic shadowing from adjacent rectosigmoid bowel.

What about this case?
Again, we always wanna do our measurements.
So here you put a little caliper here and then that's the other end.
You put a caliper here and how do you measure it?
Well, some people might just draw a line because that's easy, but is it good?
Absolutely not. That line measures 28 millimeters and I'm sure you realize that is not the course that her cervix is taking.
That's a linear measurement and her cervix is curved.
So you've gotta take the time to trace out the way that cervix really the end of cervical canal, the course of it.
And in this case it's 42 millimeters. That's appropriate.
The linear measurement of 18 millimeters, that is inappropriate.
They don't equate, take the time, look at the course of the endo cervical canal and measure it.
I think that this configuration of the curse curve cervix is not that much of a problem in the third trimester
when the cervix is actually I think more linear in most cases.
But sometimes in the second trimester and after all, women who have incompetent cervix,
that's the time you might wanna be following these women and just don't, don't be sloppy.
Put your calipers on appropriately. Get a good measurement.

So some of the pitfalls that would be false positives, meaning that you think it's abnormal but it's really not it's normal
would be times when you could have what looks like endo cervical fluid.
The bladder can cause that, you can have fluid from ne boan cyst or vaginal fluid.
We talked about lack of cervical mucus and the big one is shadowing from adjacent rectal sigmoid bowel
causing you not to be able to see the external loss when you're doing a trans labial scan.
The curved cervix is one other pitfall from a technical measuring point.

Evaluating the Ravi Cervix: Approaches Summary

Okay, so when we're talking about evaluating the Ravi cervix, we've already said transabdominal scan done most common,
but not as good as the other two ways of looking.
Trans labial scan is good, but it has the problem particularly with the external loss,
giving us some limitations when we have to measure its length.
Transvaginal is the best.

Placenta Previa Diagnosis

Now I'm not gonna spend too much time about diagnosing placenta previa, but I wanna make some important points.
One is we're good at for making the diagnosis.
Two is that we over call it, we have false positive diagnoses.
So our specificity is clearly under 100%.
And what are the problems that we have relate to technical factors?
Okay, we already addressed how full the bladder is.
That's a technical problem.
The degree of previa and the gestational age.
I'm sure most of you realize that we particularly overcall earlier on in the second trimester
and then the cervix seems to or the placenta seems to move away later in pregnancy.
So if you're gonna be diagnosing previa in the second trimester, make sure you get a follow-up
because there are many cases later on when the cervix is no longer a previa.
Also from a technical point of view, we talked about the bladder distention.
Realize that the cervix itself is almost like, oh, let's say a roll of paper towels, okay?
It has an anterior component posterior and two lateral components.
That's all the towels against the cylinder of the paper towels.
Whereas the endo cervical canal, and if you were just to do a sagittal scan
and you were insisted on looking at the midline, you could miss a placenta that's coming in from the side.
This is a transabdominal approach and we're looking at a transverse view of the cervix.
So in this case, the placenta is on the left side of the uterus and it's clearly coming upon the ipsilateral cervical wall,
not crossing over the internal US but on the cervix per se.
And if all you did was a sagittal midline scan, you would miss that.
So make sure you scan from multiple planes and go through the entire cervix.
So this would be a placenta that we would call a a marginal placenta previa.
I'm sure many of you have noted, and we've already mentioned that the placenta moves with time.
Here's an example where there's a posterior placenta there and it ends here.
This is a trans labial scan and this is the endo cervical canal.
So this placenta is completely crossing over the cervix.
It's a placenta previa.
But now six weeks later, the same case again, a trans labial scan.
Now the placenta is ending here and the internal loss is here.
Anterior, posterior wall, the cervix here.
So now this placenta is just coming to the edge of the posterior wall of the cervix.
Some people might call that low lying.
If this measurement is less than two centimeters, that is the term that we use today for a low lying placenta.
So we've gone from a complete preview here to a low lying placenta in six weeks.

Well, what is going on here?
What is causing this, this, a wonderful concept was termed by a Dr. Beka and he is a placental pathologist,
believe it or not, the University of California San Diego.
And he coined this process of the moving placenta due to a concept, he called it tropho tropism responsible for migration.
Well, what exactly is this concept?
The concept is that the placenta likes a good blood flow and it actually has been shown to proliferate
where there is a good blood, blood flow and atrophy where there is a poor blood flow
and the lower portion of the uterus does not have as good a blood supply as the upper portion of the uterus.
So with time, if you have a placenta that was in the lower uterine segment or even over the cervix
with time because there's poor blood supply there, it tends to atrophy and it tends to proliferate on the opposite side
growing up toward the upper body and feic portion of the uterus.
So let's look at this process.
Schematically a normal placenta is a discoid structure.
The cord in most people starts centrally as we see here.
Now let's say that this portion of the placenta was in an area of blood, poor blood supply.
Let's say it was over the cervix with time, that portion of the placenta might atrophy as we're seeing here in white
and going up toward the upper uterus.
It would proliferate here.
The cord does not change its location, it stays here.
And what I really like about this concept, not only does it explain the quote migrating placenta,
but it also explains some placental anomalies such as when the cord inserts now at the margin of the placenta,
that is called a marginal cord insertion.
Some people have called that a so-called battle door, placental appearance.
So if this continues this process, let's say now we're still over the cervix here
and we continue to have atrophy and concomitant, growth of the placenta up high.
Now you might have a situation where the cord is not even on the placenta any longer.
It is on the fetal membranes that are here, but it is not on the placenta.
Well, what do you call this?
We call that marginal, what do you call it when the cord inserts on the fetal membranes but not on the placenta?
You call that a veis cord insertion.
And this is a very important concept because if this situation occurs and you have a veis cord insertion
and these membranes now are overlying the cervix, you call that a VA previa.
We're gonna talk a bit more about VA previa, but it is a very, very important situation
because if a woman attempts to deliver a baby in this situation through a vaso previa,
that baby has about a 50% chance of exsanguinating.
And that is something that is exceedingly dangerous, therefore, so this is what a veis cord looks like.
The cord itself is not attaching directly onto the placenta, it attaches onto the fetal membranes.
And as you can see here, it breaks up as we see here into various vessels, to a variable degree.
So this is mentis, and if this is over the cervix, it's a VAs previa.
Every VAs previa is veis, but not every mentis situation is over the cervix.
So therefore, not every mentis situation is a VAs previa.
There are even websites that address this.
And you can see here that this placenta has veis cord coming over the cervix, IE EVAs previa.

So what is the take home message here?
If you think about it, this woman earlier in pregnancy might have had a placenta previa.
The good news is, if you wanna think about this, the good news is the placenta migrated.
She no longer has a placenta previa, but the very, very, very bad news is that she may be left with a vasa previa,
a bad, bad situation.
So a migrating placenta is not always a good thing.

Diagnosing Vasa Previa

So how do we make this diagnosis?
Well, you have to be wary of a migrating placenta previa.
You have to be very in tune to where does the cord insert on those placentas that appear to migrate.
You really need to use, I think, color doppler to look for the placental cord insertion site.
And I'll show you this in just a minute.
If you see cord vessels in front of the cervix, it's diagnostic for vaso previa.
What do I mean by cord vessels? It's not hard to determine.
You put your doppler on, you determine what is the heart rate, or you put your M ODed on,
determine what is the heart rate in those vessels and you compare it to the fetal heart rate.
And if they're one and the same, those are cord vessels.
Okay? So here we have an example.
You can see the placenta here, you see what looks like cord here and you determine those are, that's the cord.
And you see you determine the heart rate there.
It's the same as the fetal heart rate.
That is a vaso previa.
And here is the cervix down here at this level.
So that is right over the cervix.
Here's a different patient.
There is a breakup of the cord into its various membrane components, but it is not over the cervix.
So this is a mentis cord that's breaking up and it attaches somewhere else to the placenta.
These vessels do, but it's not over the cervix.
So this is veis and this is veis, but over the cervix.
Hence we use the word vaso previa.

So let's look at a few examples.
Here's a patient who has a placenta.
Here's the cervix, there's a little vessel there.
Well, what is that vessel?
You have to determine if that is going at a fetal heart rate.
You put your doppler on in this case it's going at a fetal heart rate.
That is a vaso previa.
So even though it looks pretty innocuous here on the color image, it's not at all innocuous.
This is a vaso previa.
This lady needs to be delivered by cesarean section.
Even if the placenta disappears and is no longer on the cervix,
the fact the cord vessels are on the cervix makes her non-deliverable from a vaginal point of view.
So she needs to be sectioned.

Now remember a while ago I said, when you're gonna measure the cervix, I said, you wanna pull the presenting part up
so it's not overlying the internal loss.
You'll get a better measurement, you'll see the internal loss better.
And there's another reason why this lady actually had a placenta preview and then she was told her placenta migrated good news,
no longer has a placenta preview.
Then she came back in with a little bit of bleeding.
And when we looked more carefully, that is by pulling the place, presenting part all away from the internal OS area,
which would be here.
We noted a little vessel here that you could never see when the presenting part was up against the internal aspect of the,
the internal OS of the cervix and plastered against the cervix itself.
So we started getting concerned, we looked harder and we were concerned about this.
And I'll show you a clip in a minute. There you go.
There's a vessel that's coming completely over.
You can see over the internal loss, over the entire cervix.
And then what you wanna do is to determine what is the heart rate in that vessel?
What is the, how many beats per minute?
This is it how fetal heart rate. This is a VAs previa.
Here's an example where that patient had been told everything was fine, her placenta had migrated
and nobody made the effort to see what was going on over her cervix.
And that is so critical.
I know I'm emphasizing this point over and over, but it's critical because if that baby had, you know,
an attempt to deliver her from below, she would have about a 50% chance of exsanguinating.
Because as these vessels tear, this is fetal blood that is she's bleeding out fetal blood
and the baby has a high rate of exsanguination.

Can you ever have an error and think it looks like a vaso previa but it's not?
Well, here's an example. This is the cervix.
Here's the fetal head, and here is these are cord vessels.
You can see the, you know, the heart rate here is going fast.
It's the same as the fetus cord vessels, but this is a sneaky case because this is not the cord fixed over the cervix.
As a matter of fact, in this woman, when you looked, where's the cord inserting?
Her cord is inserting normally on the posterior aspect of the placenta.
And she had a cord that was just sort of wandering and was in front of the head of the baby.
You know, the cord can move around the floating cord and in this case it was just in front of her cervix.
This lady was not in labor.
A few minutes later that cord moved out of the way.
Now, if she was in labor and the cord is the presenting part, that lady also needs to be delivered by a c-section
because you don't want the cord to come down before the head and get compressed and the baby become an ic.

On that point, I think an important take home message is if you see a placenta previa,
let's say you're scanning a woman, you're doing a 16 or an 18 week survey scan, she's got a placenta previa.
What I suggest you do is make an effort in that woman to see where does her cord insert.
If her cord is inserting, as we see here, high up way away from her cervix,
even if her placenta starts to atrophy over the lower uterine segment, she's not going to end up with a vasa previa
because her cord is not in danger of being over the cervix.
But if a cord is low down where it inserts and she starts to migrate, she can be in trouble.
And the reason I suggest you spend some time looking for the cord insertion, let's say it's 16 or 18 weeks,
is it's much easier to determine where it's inserting at that point in pregnancy,
then waiting until she's 34 or 36 weeks.
And then the baby may be obscuring the posterior placenta in a case like this, not allowing you to determine
where the cord is inserting.
So spend a little time when you have a placenta previa in the second trimester to try to determine where the cord is actually inserting.

Here's another example.
Cervix looks like, you know, vessels for sure, but when we put the doppler on in this case,
all we could see was venous flow.
And what this represents is what we call the marginal vein Around the placenta, there's a large vein that surrounds the placenta.
This is a placental vessel, but it is not a cord vessel.
And so a woman who has this doesn't actually have a vaso previa.
Now if, if she's in labor, again, this is where her placenta ends at this point where the marginal vein is,
it's not down here where the cervix is, where the placenta looks like it's ending.
You include that vessel as the end of the placenta.
So she in this case has veins that are dilated.
These are maternal vessels, they're not fetal vessels.
If she bleeds, it's from maternal blood.
The baby doesn't have, as you know, high a chance of exsanguinating.
It's not good. You do not wanna deliver this woman either from below, but sometimes these regress
and the placenta ends up just fine.
This is not a VAs previa.
If regresses and that marginal vein ends up far away from the os, she's gonna do fine.
She does not need to be delivered in that case with a C-section.

So false positive vaso previs include what we call the obligate cord presentation where it was just the cord in front of the head.
That woman was not in labor. You don't worry about it.
If she is' in labor, she needs to have a section, the marginal vein that's around the placenta.
And sometimes you just have dilated veins on the cervix, so-called cervical varis.

So with respect to evaluating for placenta previa, very often you can determine where the placenta ends with an abdominal scan.
I think a trans labial scan or probably a transvaginal scan are equally good.
You don't need to do a transvaginal scan if the trans labial scan works for you.
So I think either of these can be done.
I don't think it's necessary to say you don't do a transvaginal scan when somebody has a previous
used to be said that maybe you could precipitate bleeding.
Don't forget that bleeding is coming from up above the internal loss.
Your vaginal probe is not going up there, so don't worry about that.
You can do a transvaginal scan when you're trying to evaluate a placenta preview.

Conclusion

So I always thank Dr. Netter, whose pictures I use for diagrams and I wanna give him credit here.
And I want to thank you for your attention.