Obstetrical Measurements - Which ones are important? - HD

Introduction to Obstetrical Measurements

I am Dr. Peter Dubay, the professor of radiology at the Brigham and Women's Hospital and Harvard Medical School in Boston.

The title of my lecture is Obstetrical Measurements, which ones are Important.

For the next half hour or 40 minutes, I'll be addressing the topic of obstetrical measurements, which ones are important, and there are a number of basic measurements as illustrated on this image, the crown rum length in the first trimester, in the second and third trimesters, measurements of the head, abdomen, and femur.

And one of the, I'll be talking about how to use these, but one of the other, elements that I'll be focusing on is of the various other measurements that one could measure which ones are important to measure, especially on a routine obstetrical ultrasound out.

Key Question on Obstetrical Measurements

One of the, questions, I'll pose a question at the beginning and come back to it at the end.

In, with respect to obstetrical measurements, which one is most likely to get you into trouble?

And by trouble, I mean, which one? Based on my own experience at least, is most likely to lead to a medical malpractice, suit against you if you do it badly.

The crown rumpel length mis measuring the bial diameter, the abdominal diameter fetal femur length, nal translucency, nuchal fold, or cerebral lateral ventricles.

And again, I will come back to this at the very end.

Goals of Obstetrical Measurements

What are the goals of obstetrical measurements?

Obstetrical measurements are a very basic part of, obstetrical ultrasound.

There are many other parts, but measurements are one of the basic parts.

And the goals are to, assign gestational age, predict the outcome of early first trimester pregnancies, assess overall fetal size and growth to diagnose, abnormal growth patterns such as intrauterine growth restriction, and macro somia not only assess the overall fetal size and growth, but, determine the size, of, individual fetal body parts to determine whether they're normal to identify aneuploid fetuses.

In other words, fetuses that have abnormal chromosomes to assess the intrauterine environment and make sure the fetus isn't compromised.

And, measurements, can help determine the likelihood of preterm delivery.

Potential Measurements Across Trimesters

There are many, many potential measurements in the first trimester, in the early first trimester between about five and seven weeks.

The potential measurements include the mean sac diameter, the crown rump length, the embryonic heart rate, the oak sac diameter, and some doppler measurements, resistive indices in, some of the arteries in and around the, uterus in the mid to late first trimester.

One can measure the crown rum length or nuchal translucency in the second and third trimesters.

The number of potential measurements, that one could measure really explode many, many fetal body parts other than, in addition to the standard measurements of the head, abdomen, femur, measuring the cerebellum, cisterna, magna, and, quite a few others.

And then there are some non fetal measurements, the amniotic fluid index, cervical length, and some doppler velocities and indices.

So, as you can see between the first trimester and the second and third trimester, there are a very large number, and I haven't listed, them all here, but there are, there are a very large number.

One of the messages of this, presentation is that, at least for routine use, a very limited subset of all possible measurements should be measured on a routine basis.

And I'll be talking about, those here.

Societal Guidelines on Measurements

So what's in the, various societal guidelines?

A number of, societies have put out guidelines, including the American Institute of, ultrasound in medicine, the American College of Radiology, American College of Obstetrics and Gynecology.

They try and keep their guidelines in sync with one another.

So what's in those guidelines?

With respect to measurements, just focusing on what's in the, guidelines that have to do with measurements.

Not all of the other things in the guidelines.

In the first trimester, the guidelines say that the crown romp length should be recorded when possible, and the mean act diameter may be recorded when the embryo is not identified.

A little difference between those, the should and the may, and I'll get into those, during my talk.

The nuchal translucency, they say, should be measured for those patients desiring to assess their individual risk of aneuploidy.

What's in the guidelines?

For, with respect to the second and third trimesters?

In measurements, amniotic fluid should be assessed either qualitatively or semi quantitatively.

The gestational age, the guidelines say, one can use a variety of parameters, including different head measurements, abdominal measurements of femur length, length.

Very importantly, they say not to redate a pregnancy if an accurate prior scan has already been done.

And, fetal weight estimation, should be done, especially in the third trimester.

And a, variety of measurements, should be used or can be used to estimate the weight and, the fetal weight should be, compared to published, percentiles.

First Trimester Measurements

Mean Sac Diameter

So let's now go into, individual measurements, starting with the very earliest one, that one could measure, the mean sac diameter.

The mean sac diameter is, the average of three measurements of the fetal, of the gestational sac measurement, average of the, sagittal transverse and enter posterior, dimensions of the gestational sac.

Those are measured in millimeters and averaged in this, particular case, the mean sac diameter 16.8 millimeters.

So it's a measurement. What can you do with it?

Well, there are two potential uses.

One is assigning gestational age, between five and seven weeks.

And a second potential, use is diagnosis of early pregnancy failure.

And the currently accepted guidelines for that are that when the mean S Act diameter is 16 to 24 millimeters, and you see no embryo, you can diagnose probable pregnancy failure when the mean S Act diameter is, at least 25 millimeters and no embryo, definite pregnancy failure.

With probable pregnancy failure, you don't want to act other than to get a follow-up, scan, with a, diagnosis of definite pregnancy failure.

No more information needs to be collected.

You have the definite diagnosis, in, this is a, case of a means act diameter of 15 millimeters and no yolk sac.

At one point it was thought that this was a reliable, measurement, reliable criterion to diagnose pregnancy failure.

But, we now know that it isn't.

And here's an example where it's 15 millimeters and no yolk sack, and a week later there's an embryo with a, heartbeat.

So the, mean sac diameter without a yolk sack is not really, reliable.

So, what about dating by the mean sac diameter?

Well, there are actually two methods, two competing methods to date an early pregnancy.

One is by the means act diameter.

So in a case like this one here, you can see the means act diameter, is an average of, 8.3 millimeters, five millimeters, and seven and a half millimeters.

So mean sac diameter is 6.9 millimeters.

If you look up in one of the standard, tables, when the, mean sac diameter is 6.9 millimeters, this would give a gestational age of 5.6 weeks.

There's an alternative method of, dating in the first, trimester, in the early first trimester before you see the embryo.

And that's to date by the SAC contents, by when you date by SAC contents.

If you see a gestational sac with no, yolk sac or embryo, you had dated at, five weeks, if you see a gestational sac, with a yolk sack, but no embryo at five and a half weeks.

So, dating by SAC contents, we would date this as five weeks.

So these are two competing methods.

They give a slightly different answer in this case.

So if we look overall at the means act diameter, in favor of it, is that it's moderately accurate for assigning gestational age and fairly well studied.

But against the mean sac diameter, is that it is, there's a fair amount of inter observer variability.

If two people measure the mean sac diameter, they can get somewhat different answers.

And there is an alternative approach that may be superior for assigning gestational age.

The way that we use it, our own facility, is if we see a gestational sac and yeah, no yolk sac or embryo, we call it five weeks gestational sac with a yolk sac by no, but no embryo, call it five and a half weeks.

If we see a gestational sac and an embryo, then we measure the crown rum length to assign the gestational age.

So you don't need to use the mean sac diameter for pregnancy, dating.

And in many cases, it's not definitive for diagnosing pregnancy failure.

So it's an optional, not essential measurement.

Crown Rump Length

What about the crown rump length?

Crown rum length here we're seeing, in very early pregnancy, there's the gestational sac, a little round yolk sac, and there's an embryo.

We know it's an embryo with a heartbeat, and we're measuring it at about 2.7 millimeters.

That's one me, that's a measurement.

Early in the first trimester, in the late first trimester, you can actually see the different fetal body parts, and you're measuring it from the top of the head or the crown of the head to the bottom of the fetus or the rump.

So it's the length of the fetus minus the, without the lower extremities.

What are the potential uses of the, crown rum length?

They are assigning gestational age at, six to 13 weeks, and also diagnosing early pregnancy failure.

Anytime the crown rum length is at least seven millimeters and now heartbeat, you can diagnose definite failed pregnancy.

And here's an example here is, an embryo measuring 7.1 millimeters.

It's at least seven millimeters on this realtime video clip.

That's no heartbeat. This is a definite pregnancy failure.

Whereas, in this case, we have a, roughly four millimeter embryo seen here.

There's no heartbeat.

This is probable or suspicious for pregnancy failure, but a follow-up would be needed to confirm that likely diagnosis.

So the crown rum length is accurate for assigning age, provides a diff a definitive diagnosis of early pregnancy failure.

It's extensively studied, and there's really no, nothing against its use.

It should be measured routinely at six to 13 weeks.

Embryonic Heart Rate

What about the embryonic heart rate?

This is, something that can be measured at any time during pregnancy, via m mode.

As we'll see in a minute, it's most useful between about six and seven weeks in the early first trimester.

Here you can see a heartbeat that looks a little slower than you expect to see.

Here's one more in the range that you do expect to see.

When we measure these two by M modes, you can see the one, the first one that I showed you is, going, the heart is going at 80 beats per minute, the second at 120 beats per minute.

So what's the, what's the benefit or potential use of the embryonic heart rate in the early first trimester?

Well, the potential use is predicting the pregnancy prognosis.

If we see heartbeat, we know that the embryo is living at the time of the scan, but it gives us, valuable information about pregnancy. Prognosis.

The, relationship, there's a clear relationship between the, embryonic heart rate, and, survival rate.

And as you can see, when, very early on, up to about 6.2 weeks or crown rum length up to, to four millimeters, you can see that when the heart rate is less than 80 virtually, there's virtually no chance that, the fetus will survive till the end of the first trimester.

At 80 to 89 beats per minute, there's about a one in three chance that it'll make it about a two in three chance if it's in the nineties.

And the, likelihood of first trimester survival plateaus when the rate hits a hundred.

So a hundred can be considered the normal, a hundred or above normal with, very early first trimester.

With the crown rum length, slightly bigger, five to nine millimeters, all of these numbers go up by 10.

When the, if you have a 5, 6, 7, 8, 9 millimeter crown rum length and the heart rate is less than a hundred, then the, there's virtually no chance and it plateaus at 120.

So we can, in terms of classification, classify the heart rate as slow if it's less than 90, up to 6.2 weeks, or up to a, crown rum length of four millimeters normal.

If it's above a hundred and borderline in between, with those numbers being 20 beats per minute, higher at 6.3 to seven weeks.

So the heart rate is good prognostic indicator, there's nothing against it, and it should be measured routinely, at six, to seven weeks.

Yolk Sac Diameter

Another thing that can be measured early on is the yolk sac diameter.

Why bother? Well, again, it has, prognostic value in diagnosing early pregnancy failure.

If the diameter is greater than six millimeters, there is probable pregnancy failure.

And here are a couple of examples.

Here is an embryo sitting beside the yolk sac.

This is the yo sac, not the amnion.

And, you can see the yolk sac looks big.

We put calipers on it.

It's measuring big nine millimeters, six millimeters as the upper limit of normal.

And this went on. It's a very, worrisome finding.

Even though the embryo is alive at this point, there's an elevated risk of subsequent failure 'cause of the large yolk sac and the embryo, the pregnancy went on to demise on a follow-up scan a week later.

So the yolk sac diameter is moderately reliable predictor of pregnancy failure when it's large.

It's not often an isolated, finding indicating failed pregnancy.

And the recommendation here is that when the yolk sac is greater than six millimeters in diameter, one should do a follow up scan.

It's not very often a useful measurement, but if it looks big, we measure it.

Nuchal Translucency

What about the nucle translucency potential uses here are screening for an or abnormal chromosomes, and in and of itself, it gives some information fetuses with an elevated, with a nucle translucency bigger than roughly two and a half, millimeters at 11 to 14 weeks have an elevated risk of aneuploidy, especially Trisomy 21 or Down Syndrome.

And it gives information about pregnancy prognosis beyond aneuploidy.

And that fetuses with, a, thickened nal translucency, especially above three millimeters, and an even if karyotype is normal, have an elevated risk of adverse outcome, including pregnancy, spontaneous loss, and, cardiac anomalies.

Very important to measure the nal translucency, accurately, because it's such a small measurement, we, there are very, specific criteria for its measurement.

I'm not gonna go through each of these here, but you do have to follow very strict guidelines for how you measure it, including where you put the calipers.

If this is the dark space, that's the nal translucency, you wanna put the calipers exactly just on, be on the outside of this dark space to get the measurement.

So, I can tell you that there's a difference of opinion, of opinion with respect to whether the nal translucency should be routinely used, but we certainly use it as a routine part of our obstetrical protocol on all scans performed at 11 to 14 weeks.

Even if the patient isn't having an associated blood test, because, thickening of the nuchal translucency may prompt further testing for aneuploidy, including some of the serum tests or CVS or amniocentesis or, and a follow-up scan for the risks other than aneuploidy.

We also offer a comprehensive program called a First Look program that combines the nucle translucency measurement and maternal serum screening.

Here are a few different fetuses, all with thickened nal translucencies.

One had trisomy 13, one went on to have a diagnosis of Trisomy 21.

One had demise two weeks later, and one, even though the NAL translucency was very large, was normal.

So remember, nal translucency is an indicator of possible or probable problems, but, not death.

So, in favor of the NAL translucency is that it provides information in early pregnancy about the risk of annuity and other pregnancy complications.

Against it is that information about aneuploidy is less reliable if it's not used in combination with serum markers.

We recommend using it routinely at 11 to 14 weeks using strict criteria, ideally done in the setting of a comprehensive ultrasound and serum marker program.

Second and Third Trimester Measurements

So let's go on to the second and third trimester.

There are some, what we might call standard fetal body parts, the head, abdomen, and, femur.

As seen here, you have to measure them carefully.

Measure from leading edge to leading edge to measure the viridal diameter.

Make sure that you have the internal structures, the thalami, the Cajun septum, lucidum in view, ideally at least.

And, measuring the OFD, the occipital frontal diameter or length of the head from the mid skull to the mid skull.

For the, abdomen, we wanna have a nice round abdomen seeing all or almost all of the, skin surface around it and put the calipers on the skin surface.

Not for example, on the ribs here or here, but right on the skin surface and the measurement of the femur.

You want to see a nice, clear cut, bone with clear cut edges.

So what are the potential uses assigning gestational age, estimating fetal weight to diagnose fetal, size problems, diagnosing structural anomalies such as skeletal dysplasias, if the femur is short microcephaly, if the head small and screening for aneuploidy.

I'm not gonna say too much about these other than to say that they play a critical role in the assessment of gestational age and fetal growth.

They're extensively studied, nothing against them.

They should be measured routinely in the second and third trimesters.

Routine Use for Gestational Age and Size

And what do we do routinely?

Well, in, the, for gestational age and size on a first trimester, on the first sonogram in a pregnancy, we estimate the gestational age after the first sonogram in a pregnancy.

We never red date the pregnancy because we always, we take the, prior the gestational age that was assigned at the prior ultrasound, add the number of intervening weeks and get today's age.

On the, so again, on the first sonogram in a pregnancy, when we, the one that we do estimate the age, if it's prior to six weeks, we base the gestational age on the sac contents or the mean sac diameter between 16 and six and 13 weeks.

We base the gestational age on the crown rump length.

And after 13 weeks, we base the, our, assessment of gestational age on head measurements.

And we use a, one of two head measurements.

But we want to use a, measurement of the head that takes into account both the width and length of the head.

So, and there are two ways to do that.

One is to measure the head circumference by putting ca uh, electronic calipers around the, outside of the skull.

And another is to measure the occipital frontal diameter and, sorry, the bi parietal diameter and occipital frontal diameter.

And, get what we call a corrected biparietal diameter.

Using this formula, this formula makes sure that if you have an a, an average shaped head, the corrected biparietal diameter is exactly the same as the biparietal diameter with a long head.

The corrected BPD is larger than the BPD alone.

And with a short or dolichocephalic, brachycephalic head, the corrected BPD is less than the BPD.

And then we use standard BPD formulas.

The other thing we do routinely, with respect to gestational age and size is in the third trimester measure or estimate the fetal weight.

We estimate the weight via measurements of the head, abdomen, and femur, and then we assess the fetal size in relation to the age via the estimated weight percentile.

We look at, but don't pay too much attention to ratios like the femur length to abdominal circumference and head circumference to abdominal circumference ratio.

When we're assessing fetal size in relation to the age, we focus almost completely on the estimated weight percentile.

Anytime the estimated weight percentile is less than the 10th percentile, we're worried about growth restriction.

And then that sets off a set of, actions to try and attempt to determine the cause.

And to monitor the fetal closely, the calipers for the abdominal, diameter or circumference must be on the skin surface.

So here, for example, as the sin skin surface, I sometimes see people mistakenly put it on the, edge of the rib as opposed to the edge of the, skin surface.

This is correct, this is incorrect.

The other important message, I sometimes see people try to measure an abdominal diameter or circumference when you can't see the skin surface.

This is about the worst case that I've seen where somebody actually put the calipers on this structure, even though I have no idea where the skin surface surfaces here, they should not have measured the, abdominal circumference here.

Cerebral Lateral Ventricles

Another second or third trimester measurement is the cerebral lateral ventricles.

We measure that as the width of the ventricle from the medial wall of the ventricle to the lateral wall of the ventricle, medial wall to lateral wall.

And the appropriate place to measure it is right at the posterior aspect of the choroid plexus.

Choroid plexus sometimes, dangles down a little bit.

You don't want it to dangle down too much.

The beauty of this measurement, the cerebral lateral ventricle measurement is that in the second and third trimester, the upper limit of normal does not change during pregnancy.

It's the only measurement of the fetus that fits that pattern, and that's, the upper limit of normal.

And the second and third trimester is 10 millimeters.

So this one's normal, and this one is abnormal hydrocephalus.

So the potential use is diagnosing ventricular magaly or hydrocephalus.

Normal less than 10, mild ventricular magaly is 11 to 12, and moderate to severe is at least 13.

It's accurate. And for diagnosing ventricular magaly, it's a quick and fairly easy measurement, nothing against it.

And one should always assess the ventricles, either assess them subjectively and measure if it's equivocal or abnormal, or just measure routinely in the second and third trimesters.

Renal Pelvis

Yet another measurement is measurement of the renal pelvis.

Here the key measurement is the interop posterior diameter on a transverse view of the fetal abdomen.

As we're measuring here, here, and here, these are normal.

One is abnormal and one's normal, and one is large.

What is large?

Well, we use it for diagnosing hydronephrosis, and we consider anything less than five millimeters, to be normal, slightly less at 16 to 20 weeks, less than four millimeters.

Equivocal is five to nine millimeters, slightly less at 16 to 20 weeks, and abnormal is at least 10 millimeters.

We also use it to diagnose, or to help di detect, trisomy 21.

When you have mild bilateral hydronephrosis, it's considered a minor marker for, down syndrome.

So in favor, it's a good screening test for hydronephrosis may lead to postnatal evaluation and management that preserves renal function.

Nothing against, and same recommendation as for the, lateral ventricles assessed subjectively and measure if equivocal or just measure routinely.

Nuchal Fold

NCAL fold is another important measurement here.

We measure it from the back of the, skull to the edge of the, skin surface, again, back of the occipital skull to the skin surface.

And the potential use here is diagnosing, aneuploid.

It's, often considered a major marker 'cause it's, a, a strong indicator of down syndrome if it's abnormal.

Anything under five millimeters is normal.

At least six millimeters is abnormal.

With, about a 10 to 20 fold, increased risk of down syndrome, and five to 5.9 millimeters is borderline.

It's a useful screening method for Trisomy 21.

Quick and easy, it should be measured routinely at, 16 to 20 weeks.

Nasal Bone

Another potential measurement is the nasal bone, which you measure on a midsagittal view.

Here, it's normal Here you just see a little.in a, fetus with Trisomy 21, its potential use is in diagnosing Trisomy 21.

And the diagnostic criteria are either an absent or short nasal bone.

There are some norms as listed here.

It's potentially reliable, but it is prone to under measurement if you're off axis and may be unmeasurable.

If the fetus is prone.

I really think of it as an optional measurement.

Amniotic Fluid Index

Another measurement is the amniotic fluid index measuring in four quadrants, right upper quadrant, right lower quadrant, left upper quadrant, left lower quadrant, add together these measurements and you get an amniotic fluid index of 12.4 millimeters.

It's use is in diagnosing, oligo and polyhydramnios.

Anything less than five millimeters considered by most of the oligo amniotic fluid index above 18 to 20 millimeters centimeters polyhydramnios.

So it's moderately accurate, fairly well studied, but against it is that subjective assessment may be as good or even better.

We in our, facility use subjective assessment, so always assess amniotic fluid volume, but it can be done either subjectively or via the amniotic fluid index.

Summary of Important Measurements

So to sum, summarize which measurements are important, routinely measure the crown rump length, the, early, embryonic heart rate at, six to seven weeks, the al translucency, in the second and third trimester.

We routinely measure the head, abdomen, and femur to, we assign gestational age at the time of the initial scan, estimate the, fetal weight in the third trimester determine the weight percentile.

We measure the NAL fold at 16 to 20 weeks, the cerebral, lateral ventricles and renal pelvis.

We either measure or assess subjectively and measure if it looks suspicious or abnormal.

Amniotic fluid index or subjective assessment should always be done.

I didn't get into it here, but, we measure the cervical length, especially at 20 to 30 weeks.

And then there's some other things that we measure in specific situations.

The, if the, femur is markedly short or deformed, we measure all the long bones.

If we're doing a targeted aneuploidy scan, we would, look at the nasal bone and measure at the humerus fetus who's at high risk for intrauterine growth restriction.

Measure the umbilical artery systolic to diastolic ratio and potentially the middle cerebral artery puls ity index.

And in cases of su, suspected fetal anemia, when there's ISO immunization or hydrops, we measure the me metal cerebral artery, peak systolic velocity.

Conclusion: Measurement Most Likely to Cause Trouble

So I started out by asking which of these measurements is most likely to get you into trouble if you do it badly?

And the answer is the abdominal diameter.

I've seen a number of, medical malpractice cases where the, that, where the error, that led to a bad outcome was a mismeasured abdominal diameter leading to underestimation of the fetal weight leading to a, attempt at a vaginal delivery when a cesarean section should have been done, should have been done.

And, here are actually two cases that I've seen where the abdominal diameter circumference was, mismeasured under measured, leading to, abnormalities that arose because of a, inappropriate attempt at, vaginal delivery.

So I've come to the end of the tour of, obstetrical ultrasound measurements and, indicated which ones I think are important.

Thank you.