Obstetrical Measurements - Which ones are important?

Obstetrical Measurements: Which Ones Are Important

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.

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.

A Key Question

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

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

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.

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.

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.

Measurements can help determine the likelihood of preterm delivery.

Potential Measurements by Trimester

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.

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

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 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.

I'll be talking about those here.

Societal Guidelines

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.

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.

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

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.

Individual Measurements: Mean Sac Diameter

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.

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

There are two potential uses.

One is assigning gestational age between five and seven weeks.

A second potential use is diagnosis of early pregnancy failure.

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.

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.

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

The mean sac diameter without a yolk sack is not really reliable.

What about dating by the mean sac diameter?

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

One is by the means act diameter.

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.

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.

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.

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

These are two competing methods.

They give a slightly different answer in this case.

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.

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.

You don't need to use the mean sac diameter for pregnancy dating.

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

It's an optional, not essential measurement.