Obstetrical Measurements - Which ones are important?

Crown Rump Length

What about the crown rump length?

Crown rump 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 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 no 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 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 rumbling up 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 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 yolk 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 is 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 anate 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 nuchal translucency, especially above three millimeters, and an even if they're 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 nucle translucency, you want to put the calipers exactly, just be on the outside of this dark space to get the measurement.

So I can tell you that there's a difference 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 nucle translucency may prompt further testing for including some of the serum tests or CVS or amniocentesis or 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 aneuploidy 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.