The Cervix in Pregnancy - HD

Introduction

My name is Dr. Roya Soha, and today I am gonna talk to you about the evaluation of the cervix in pregnancy.

And I'm gonna cover, why do we even look at the cervix? When should we look? How do we look? And what do we do when the cervix is short?

And I'm here to tell you that a lot of this is controversial with the exception of how to look, which is great because we're gonna go over the ultrasound technique of evaluating the cervix.

Why Evaluate the Cervix?

Basically, evaluating the lower uterine segment is part of every second and third trimester exam. Mostly it's to rule out low lying previa or low lying placenta, and we have to perform transvaginal ultrasound if the lower uterine segment is not seen well enough with transabdominal technique.

Also, we wanna assess the cervix specifically by transvaginal ultrasound if the patient is at high risk for preterm birth.

Here's an example of transvaginal ultrasound showing the cervix and also showing that there's a low line placental with placental veins near the internal O.

Here's an example whereby transabdominal technique there looks like the place, the cervix may be somewhat short. Transvaginal exam is performed, and indeed the cervix is exceedingly shorter than it should be measuring eight millimeters in this case.

Also, the reason we look is because a short cervix is a consistent and reliable risk factor for preterm birth.

Preterm birth is a real problem, especially in the United States. Preterm birth is defined as delivery less than 37 weeks. Gestational age, 10% of births in the United States were preterm in 2014. This is data from the CDC.

Preterm birth is the leading contributor to infant death and long-term neurologic disabilities in children with an estimated annual cost of $26 billion in the United States.

Normal Cervical Length and Risk Factors

What is normal and what is short when it comes to cervical length, the one number to remember is 25 millimeters or 2.5 centimeters.

The normal cervical length is gestational age dependent. It is at the 50th percentile at 32 weeks, and it is otherwise considered short with at any other gestational age.

Normal cervical length is between 25 and 50 millimeters at 14 to 30 weeks. It's not really helpful to measure the cervix before 14 weeks, and it's considered short if it's less than 2.5 centimeters between 14 and 24 weeks.

And the shorter the cervical length, the higher the risk for preterm delivery.

However, the a priori risk also matters. A patient who has a cervix less than 25 millimeters is at 18% risk for preterm birth if she's a low risk patient. However, if she's high risk or is carrying a pregnancy with multiple fetuses, then that risk goes up to near 60%.

In addition, the shorter the cervix, the worse her risk. And if there's progressive shortening, for example, for every millimeter of shortening in between 24 and 28 weeks, there's an additional 3% risk for preterm delivery.

It becomes important to stratify the risk and know who the high risk patients are. The biggest risk factor for future preterm birth is a patient who has had a prior preterm birth, especially if it occurred early on in the second trimester.

Also, women who have had prior cervical surgery or mulian duct anomalies or multiple gestations are at risk.

The bottom line is we wanna perform transvaginal ultrasound to look at the cervical length in high risk patients in mid gestation.

Ultrasound Technique for Evaluating the Cervix

Which brings us to how do we perform this test? The technique is very important in the high risk patient.

The cervix should be assessed first at the time of fetal imaging. This is the patients are usually outpatients. They've been up and about, the cervix has been tested, and therefore should be evaluated immediately after they have been walking around basically.

We have the patient empty her bladder, come into the ultrasound suite, place the transvaginal probe, we find the midline sagittal plane and put the probe right up to the cervix.

Then we wanna withdraw the transducer until the cervix is just in focus, avoid excessive pressure with the transducer. And I'll show you why. On the next slide, we get a nice image of the cervix where it fills 75% of the screen and we measure from the internal to external os.

It's also important to watch for about three to five minutes because the cervix is a muscle as well and can change and often does when it is abnormally short. We apply fundal pressure for about 15 seconds to further test the cervix.

Here's an example of a case where too much pressure has been placed on the anterior lip of the cervix, and therefore you could see the anterior lip is much thinner than the posterior lip. And when that pressure is reduced, we can see internal os funneling in this case. And the cervix indeed is short.

We have to make sure that we don't press too hard on the cervix.

Here's a case which fits the rule of don't rush the exam. This was a low risk patient, but there was thought that there is fluid going into the cervical canal and therefore a transvaginal ultrasound was performed.

We could see that there's debris near the internal o and the cervix initially looks pretty long, however, three minutes later, the cervix is down to 15 millimeters. We know that this is abnormally short.

Now she was put into the category of a high risk patient with a short cervix, and she came for follow up five days later and the cervix is wide open. And then you can see more of this sludge and debris again, even though it is difficult sometimes for the patient and for the operator, we wanna watch for a nice period of time.

Here's an example of a short cervix, and you could see the fetal foot going in and going right into the internal OSS of the cervix. And this is difficult to watch, but it's an important information to know.

Measuring the Cervix

How do we measure the cervix? When the cervix is long and closed, it's easy and we merely measure from the internal OS to the external os.

When the cervix is abnormal and there is internal OSS dilatation, then the measurement becomes a little bit more complicated. You want to measure the complete cervical length. In addition, the diameter of the internal OSS distension and the length for at which the membranes are bulging into the cervix as well as the closed portion of the cervix.

These are the three measurements that are important. For example, the report might read the cervix measures 30 millimeters. There is 10 millimeters internal oscultation with funneling of membranes, 20 millimeters into the cervical canal. The closed portion of the cervix, also known as the functional cervix measures 10 millimeters.

Funneling more than 50% into the canal is most significant finding 80% of those patients will go on to have a preterm delivery.

It's a good idea to have a picture such as this, which you can get on this website up in the room to remind sonographers and the readers of ultrasound what to put in the report and how to measure the cervix.

Measuring a Curved Cervix

What if the cervix is curved? Then we have a few choices. Do we still measure a straight line across or segment the cervix and add these distances? Or you can even just track and draw the cervix along the curvature.

What's most important is whichever method you choose is to consistently use that method when patients come for follow up.

Other people say, some people say that if the peak angle between this line and these two lines put together is more than one centimeter, some people use five millimeters, then maybe it's best to do the additive measurement. But again, if that's what someone does on the follow-up, we want to do the same thing.

Patterns and Additional Findings in Cervical Evaluation

Interestingly, incompetent cervix follows a predictive pattern of progression. The normal cervix relationship with the internal OS is straight across. So there was a T pattern.

And then as the cervix starts to open, it opens from the internal OS initially in a V type pattern and then into a morphology that looks more like a U.

Another interesting factor that we see is that there can be sludge or amniotic fluid debris near the internal oss. This is probably a marker for intra amniotic inflammation or infection. Some suggest that it is an independent risk factor for preterm birth.

The cervix that has funneling plus debris is at a higher risk for developing into a more significant cervical opening, as in this case as the case that maybe does not have debris.

In Dr. Romero's lab in 2008, a very elegant study was performed where a needle was placed into sludge and the debris was placed under an electron microscope. And they looked at what the sludge is actually made up of and they found bacteria embedded in an amorphous material and inflammatory cells.

This microbial biofilm was unique to this tissue, was not seen otherwise in the vaginal cuff. And it is again, more proof that this is probably getting, having a short cervix is most likely related to some chronic infection or inflammatory change.

Management of a Short Cervix

Once the cervix is found to be short. What do we do? Transvaginal ultrasound surveillance is typically performed every one to two weeks in high risk patients or when the cervix is seen to be short between 16 and 24 to 28 weeks.

Non-invasive treatment is quite controversial and includes vaginal or Im route of progesterone administration. And again, this is an ongoing active area of research. More recently, pessary has been placed and we'll talk a little bit about that and it's pretty much agreed upon that activity restriction and bedrest is really not shown to help.

Progesterone Treatment

Progesterone is known to have a quiescent effect on the myometrium. Vaginal progesterone has been shown in most studies to reduce the rates of early spontaneous preterm birth by approximately 40 to 50%. And that's regardless of the patient's pregnancy history.

Intra intramuscular progesterone seems to reduce the risk of preterm birth in women who've had a history of preterm birth, but it's not been shown to be consistently effective if the cervix is short in a low risk patient or if the patient has multiple gestations.

And again, a lot of data about progesterone is controversial.

Cervical Pessary

More recently, we've been talking about placing pessary in pregnant women with short cervix. There is a very nice article in Lancet from 2012 that reviews this procedure and its data.

Basically this is what a cervical pessary looks like and it becomes important for us who practice ultrasound because we're asked to evaluate these cases in which a pessary has been placed.

And again, from the Lancet article, we see that when the transducer is introduced, if there's a pessary, there's a significant amount of shadowing that is seen. The transducer is initially placed up to the pessary, and again, you see shadowing, but then the transducer is pushed posteriorly and tipped up where we shoot down the barrel of the cervix.

And then we can expect to see images such as this where you see the indentation of the pessary. But then we can also see the cervical canal in the study, published in Lancet, the pessary group basically did better than the expectant management group for preterm birth.

Here's an example of a recent pessary case at my institution on transabdominal imaging. The sonographer thought possibly this was the cervical length. However, we pursued with transvaginal ultrasound.

And sure enough, initially when we put the transducer in, we're met with a fair amount of shadowing. And then once we angle down the barrel of the cervix, we can see the indentation of the pessary.

And in this case, this is a failed pessary because there is a complete cervical opening. Here are the membranes and there is some sludge. And unfortunately this pessary was not doing its job.

Cervical Cerclage

What about cervical cerclage? Cerclages can be placed either transvaginally and there's several different types of stitches that are performed or transabdominally, which leads to open laparoscopic approach. And an obligatory c-section delivery transabdominal cerclage stitches are much closer to the internal os when we look with ultrasound than transvaginal stitches.

And in the United States, transvaginal stitches are used more commonly.

Rescue cerclage is different. It's placed transvaginally and it's considered a heroic measure and is performed in the setting of cervical dilatation or prolapsing membranes.

How do we evaluate the cervix after cerclage has been placed? In addition to the total cervical length and the diameter of the internal oss and the portion that is open, we do wanna also measure the length of the internal OSS to the stitch as well as the length from the stitch to the external oss.

There's a lot of numbers we wanna put in the report to help the perinatologist in evaluating whether the cerclage is doing its job.

Here are some ultrasound examples of failed cerclage. In this case we can see the stitches here, but the membranes are bulging beyond the stitch and therefore the cervix is open. And again, we can see the sludge and it's extending to the external o.

Here's a case where the cervix is closed, there's a little bit of funneling, but we want to evaluate where the stitch is. And then the anterior lip, we can see the stitch over here, but external in the posterior lip, the stitch is too far distal.

And when we turn axial, we could see that this stitch is off to the side of the cervix. And probably the cerclage is not doing its job, although the cervix is closed.

Summary

In summary, why do we look at the cervix at all? Because the short, mid trimester cervical length is a significant risk factor for preterm birth, especially if the patient is high risk.

We look at mid gestation and do surveillance when the cervix is short or if a patient is at high risk. The most important aspect of what we do here is using good technique by transvaginal ultrasound to measure the cervical length.

And then what is done when the cervix is short, is controversial. Whether the patient is put on progesterone, gets a cerclage or a pessary or nothing, is pretty much still being discussed and that is what is controversial.

However, what is not controversial is how to do a good ultrasound of the cervix. I encourage everyone to use good technique from measuring the cervical length in the appropriate patient population.

Thank you.