Obstetrical Measurements - Which ones are important?

Second and Third Trimester Biometry

Let's go on to the second and third trimester.

There are some 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 biparietal diameter.

Make sure that you have the internal structures, the thalami, the cavum septum pellucidum 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 want 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.

Potential Uses

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 Assessment of Gestational Age

And what do we do routinely?

In the for gestational age 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 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 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 six and 13 weeks.

We base the gestational age on the crown rump length.

And after 13 weeks, we base 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 electronic calipers around the outside of the skull.

And another is to measure the occipital frontal diameter and the biparietal diameter and get what we call a corrected biparietal diameter.

Using this formula, this formula makes sure that if you have 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.

Fetal Weight Estimation

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 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 is 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 ventriculomegaly or hydrocephalus.

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

It's accurate.

And for diagnosing ventriculomegaly, 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 Measurement

Yet another measurement is measurement of the renal pelvis.

Here the key measurement is the anteroposterior 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 as five to nine millimeters, slightly less at 16 to 20 weeks.

An abnormal is at least 10 millimeters.

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

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

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

Nothing against that.

And same recommendation as for the lateral ventricles assessed subjectively and measure if equivocal or just measure routinely.

Nuchal Fold

Nuchal 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 aneuploidy.

It's often considered a major marker 'cause it's 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 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.

Its 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 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 summarize which measurements are important.

Routinely measure the crown rump length early, the embryonic heart rate at six to seven weeks, the nuchal translucency, in the second and third trimester, we routinely measure the head, abdomen, and femur to 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 nuchal 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 it.

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 pulsatility index.

And in cases of suspected fetal anemia, when there's isoimmunization or hydrops, we measure the middle cerebral artery peak systolic velocity.

The Most Troublesome Measurement: Abdominal Diameter

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 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.

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

Conclusion

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

Thank you.