The Role of Measurements in OB Ultrasound Ultrasound - HD

Main Uses of Measurements in Obstetrical Ultrasound

The main uses that we think about
are determining gestational age
or estimating the due date,
estimating fetal weight and percentile.

Measurements play an important role in many aspects
of obstetrical ultrasound.

In particular, they diagnose
or contribute to the diagnosis of many things,
early pregnancy failure, as you heard,
fetal growth abnormalities, assessment
of fluid volume, and many other things.

Since I only have 30 minutes, actually 25 minutes
because the Q&A in the last five minutes,
I will cover a fair number of these,
but each one of them, very briefly to give you an overview
of some of the key uses
of measurements in obstetrical ultrasound.

Guidelines for Measurements

One of the important things to think about is,
since we want to follow guidelines of major societies,
what are the guidelines that are out there
with respect to measurements?

The American Institute of Ultrasound
and Medicine, the American College of Radiology,
American College of OB, GYN, get together on the
obstetrical ultrasound guidelines.

There's one single set
of guidelines from those various societies.

What I'll show you on the next two slides is an
excerpt from the OB guidelines that include,
that refer to measurements.

First Trimester Guidelines

In the first trimester, these guidelines say that the
crown rump length should be recorded when possible.

The mean sac diameter may be recorded when the embryo is
not identified.

The nuchal translucency should be measured in conjunction
with the serum biochemistry
during a specific age interval for those patients desiring
to assess their individual risk of aneuploidy.

Somewhat vague statement
about when it should be used, suggesting
that it should be used in conjunction with
serum biochemistry.

Second and Third Trimester Guidelines

In the second
and third trimesters, the guidelines
talk about a number, some more uses of measurements than in the first trimester,
including assessment of amniotic fluid volume,
gestational age determination, weight estimation.

They suggest that the nuchal fold may be helpful
during a specific age interval.

That's what's in the guidelines.

Let me delve into some of the uses over the next,
over the length of time of my talk.

Determination of Gestational Age

First briefly, determination
of gestational age
or estimating the due date is certainly a use
of obstetrical ultrasound.

What I'll talk about here comes from a committee
opinion that was jointly from the
ACOG AIUM
and Society for Maternal Fetal Medicine
that talked about methods for estimating the due date.

One thing that I would say is that every
ultrasound should have some, every report
of an ultrasound should have some statement about your best
estimate of gestational age at the time of the ultrasound.

How can we use ultrasound to assign gestational age?

As you'll see in a moment,
what I'm really talking about is how do we use ultrasound
to assign gestational age at the time
of the first ultrasound in a pregnancy?

'Cause that's the only time that you should use ultrasound
to assign gestational age as we'll come to in a minute.

First Trimester Methods

If you see a gestational sac
with no identifiable embryo,
which would be the normal finding before six weeks,
between five
and six weeks,
you can either assign the gestational age via the mean sac
diameter or via the sac contents.

Actually, in our institution,
we don't use the mean sac diameter for much of anything,
or especially for assigning gestational age.

If we're doing a transvaginal ultrasound
and we see a gestational sac with no yolk sac,
we say our best estimate is
that we're looking at a five week pregnancy.

If we see a gestational sac with no identifiable embryo,
but we do see a yolk sac,
our best estimate is five and a half weeks.

Once you see the embryo for the rest of the first trimester,
the crown rump length is the best method
for assigning gestational age.

The assignment
of gestational age in the first trimester is very accurate
within about three to five days, plus
or minus, which is very accurate in the second
and third trimester.

Second and Third Trimester Methods

The methods
that you can use include the biparietal diameter, the
head circumference, the, or the occipito frontal diameter,
and or the femur length.

If you can see all the way around the head,
in other words, you can either do a circumference all the
way around it or measure the biparietal diameter, the width,
and the occipito frontal diameter.

The most accurate estimator of gestational age is
to use head measurements
and to use a head measurement that accounts
for the head shape, either the head circumference
or something called the corrected biparietal diameter.

I don't have time to get into it here,
but it's a method of correcting the biparietal diameter
for the length of the head.

In many cases, especially in the third trimester,
especially in the later part of the third trimester,
it's very hard to see the entire perimeter of the head.

Often, you can measure the width, but not the length.

In those cases when the biparietal diameter is measurable,
but the occipito frontal isn't,
'cause you don't see the full length of the head,
either the biparietal diameter
or the femur length is pretty good, not quite as good
as head measurements that account for head shape,
but maybe all that you can do, the accuracy gets worse
and worse as pregnancy progresses.

It's about plus
or minus one point just over one week in the early second
trimester to a not very good plus
or minus three weeks in the third trimester.

One thing that's important
to point out is the abdominal diameter circumference is not
reliable for estimating or determining gestational age.

There's too much variability from fetus
to fetus at any given gestational age in their abdominal
diameter, for it to be a useful method for determining age.

Recommendations for Assigning Gestational Age

From the paper
that I showed on the couple
of slides back, what are the recommendations
for assigning gestational age?

At the time of the initial ultrasound, at least any,
the initial ultrasound
after six weeks,
they suggest basing the gestational age on the LMP
unless the woman is unsure of her LMP
or if she tells you her LMP.

But the LMP is inconsistent with ultrasound dating.

What do they mean by that?

If you have an age by LMP that's less than nine weeks,
and there's more than a five day discrepancy between the LMP date
and the ultrasound date,
use the ultrasound date if it's within five days, use the LMP.

Similarly, for each age by LMP, there is
an age range such that if there's more of a discrepancy than
what you see on the right hand side, use the ultrasound.

If there's less of a discrepancy, use the LMP.

Very importantly,
and this probably the most important single statement I'm
gonna make during the whole talk, don't red date pregnancies
or at least redate them in very rare circumstances.

Basically, never. In other words, so what do I mean?

If a woman comes in
and based on whatever criteria we use, we come up
with a gestational age of 10 weeks, say,
via the crown rump length.

If she comes back eight weeks later, she's 18 weeks.

Doesn't matter what your ultrasound measurements show,
she's 18 weeks, you don't red date.

So at the time of the 18 week ultrasound,
you want to make sure the body parts are appropriate in size
for 18 weeks, but don't red date the pregnancy.

Don't redate the pregnancy.

So the only time we use ultrasound
to date a pregnancy is at the initial ultrasound
during a pregnancy,
and in pregnancies achieved via
assisted reproductive technologies like IVF use,
dating derived from the technique, for example,
using the embryo age
and the transfer date in IVF patients,
not the ultrasound measurements.

Estimating Fetal Weight and Percentile

What about estimating fetal weight
and weight percentile?

The best, basically,
if you add body parts from one to two, to three to four
to five, you get increasing accuracy up to three.

If you use measurements of the head, abdomen,
and femur, you can find papers out there
that throw in fourth
and fifth measurements like the thigh circumference
and so on, but it doesn't really buy you more accuracy.

Best way to estimate fetal weight is via measurements
of the head, abdomen, and femur.

Once you have an estimate of the fetal weight
and you know your best estimate of gestational age based,
generally on a first trimester ultrasound these days,
assess the fetal size in relation to the age to get,
via the estimated fetal weight percentile.

If it's less than the 10th percentile suspect
growth restriction.

Then you have to head off on a course of assessment
for that, attempting to determine the cause
of small size and monitor closely.

How do you come up with a weight percentile?

You can use a weight table.

For example, if you have a 25 week gestational age
and an estimated weight of 600 grams, you would see
that it's between the 25th and 50th percentile.

You can use a weight table, you can use a weight chart
or use some sort of
computer calculation, either a computer
inside the ultrasound machine
or separate from it, depending on
how you do your reporting and interpretation.

Very important that the abdominal diameter
or circumference measurement when you do it
to make sure the calipers are on the skin surface.

Ideally, you want to see the skin surface around all
or almost all of the abdomen.

It should be as round as possible.

See the stomach, the portal vein.

Often the left portal vein will be in view,
but importantly is put the cursors on the skin surface,
not on a rib, which I sometimes see people do.

Really importantly, this is one
of the most important measurements I'm gonna talk about
because it's one of the measurements
that I see more often than not done poorly.

These two images, believe it
or not, here you can actually see the skin surface
and the measurement was done inside,
not on the skin surface.

Here's another one
where somebody put cursors in the middle of nowhere
and you have no idea where the skin surface is.

You can barely see that you're looking at fetal abdomen.

If you wonder where I got these cases,
those are both from malpractice cases where
the estimated weight was grossly underestimated,
leading to problems at delivery
and serious complications
after birth for the baby.

Measure very carefully
and importantly, put the calipers on the skin surface.

Diagnosis of Early Pregnancy Failure

Weight, age
and weight aren't the only things by any means
that we use ultrasound for.

You heard from the prior talk about
measurements in the diagnosis of early pregnancy failure
or miscarriage, and there are criteria for
pregnancy failure that use measurements and otherwise.

I'll focus on the next several slides.
Very briefly on the measurement criteria.

Crown rump length, seven millimeters or greater,
and no embryonic heartbeat
or mean sac diameter of 25 millimeters
and no visible embryo are measurement criteria
for definitive pregnancy failure.

Findings
that are suspicious you heard in the prior talk also include
some measurement criteria,
and some that are not measurement criteria.

There are findings that are suspicious
for impending pregnancy failure when you see a heartbeat,
some of which again, use measurements, a slow
or irregular embryonic heart rate, a large yolk sac
or a small gestational sac size,
even if there is a heartbeat, if you see any of these.

Some non-measurement criteria indicate
that the pregnancy is likely going to fail subsequently,
usually within a few days after the ultrasound.

These are other uses of ultrasound measurements in pregnancy to determine,
in this case prognosis.

Seven millimeter embryo with no heartbeat,
definite pregnancy failure, a three millimeter embryo
with no heartbeat on the clip, almost
for sure, pregnancy failure.

There's a teeny chance in this case
that the pregnancy will make it a no chance in this case,
even if I see a one
or two millimeter embryo with no heartbeat, that's a very,
very abnormal finding
because it indicates a high likelihood
of pregnancy failure mean sac diameter again,
has criteria for pregnancy, definite
and suspicious pregnancy failure.

Large yolk sac, as in this case, is highly suspicious,
though not quite definitive for pregnancy failure.

A few findings that I listed that when there is a live embryo in other words,
an embryo with a heartbeat, when you see them,
there's a high risk of subsequent pregnancy failure,
including a large yolk sac measuring nine millimeters.

Anything over six
or seven millimeters indicates a high likelihood
of subsequent failure.

Slow heart rate as seen here, measured at 64 beats per minute,
is also indicating a high risk of subsequent pregnancy failure.

One simple number to remember is
that any heart rate in the early first trimester,
less than 90 beats per minute indicates a high chance
of subsequent pregnancy failure.

Assessment of Amniotic Fluid Volume

Another use of fetal measurements of measurements, not fetal ones,
in ultrasound is in the assessment of poly and oligohydramnios here.

The recommendations that I'll come up
that I'll talk about come from a multi society panel from a few years ago that came out
with its final recommendations in this paper
that was published actually simultaneously published in three journals.

Multiple societies that were involved in this.

I was one of the panelists on it, as are some
of the other speakers at this meeting.

The AIUM guidelines for second
and third trimester say that a qualitative
or semi-quantitative estimate of amniotic fluid volume should always be reported.

We'll see in a minute what semi-quantitative methods
of assessing fluid volume means.

Sonographic assessment can be done subjectively
or semi quantitatively, semi-quantitative methods include the single deepest pocket
measurement or the amniotic fluid index,
which is simply a sum of four different measurements here, subjectively, as we sweep through,
it looks like there's too much fluid and
or you can measure the amniotic fluid index
at 37.5, which is high,
or take the largest one of these 10 millimeters
as the single deepest pocket.

I have to say, at our own, in our own department,
we rely solely on subjective measurements.

The only reason that I have an amniotic fluid index slide,
case to show is that I took it
so I could put it in the talk,
but we never, we don't actually measure it.

We do subjective assessment.

Subjective assessment is quick and efficient.

It accounts at least in a subjective sense for gestational age variation in amniotic fluid volume.

When I'm looking and sweeping through either
with my own hand and eyes
or looking at a video clip, my brain has a
an expectation for how much fluid I'm gonna see at
that gestational age,
and that's how I can subjectively assess account
for gestational age variation.

It does mean that it may be of uncertain reliability
with an inexperienced operator,
and it's best documented with a video clip.

Deepest pocket and amniotic fluid index are
semi-quantitative methods.

I'll show you in a minute why,
but they're certainly simple
and quick, especially the single
deepest pocket measurements.

There are some downsides.

The biggest one is that you can have a pretty thin pancake
of fluid that may be big in two dimensions,
but very thin in one dimension, just like a pancake
that has very little volume in it.

But when you measure the deepest pocket
or even look at it in a second dimension may look big.

Biggest problem that I have is
that they're really not at all mathematically valid.

There is no direct relationship
or even indirect relationship
between linear measurements and volume.

What do I mean by that?

Here are two human beings that have the same height, same single deepest pocket,
and they have very different volumes.

They're not great.

What did the fetal imaging workshop recommend?

Either a subjective assessment
or semi-quantitative methods are both acceptable,
either one of which,
and they recommended the single deepest pocket over the
amniotic fluid index.

Because there are a number of studies that show
that the single deepest pocket leads
to fewer fetal interventions without any significant difference in outcome.

Other Uses of Measurements

Markers of Aneuploidy

I'm not, this is a topic, markers
of aneuploidy that could take a whole lecture
or a whole day, and obviously I'm not going to,
but aneuploidy markers are findings
that increase the likelihood of aneuploidy, but have little
or no significance in and of themselves.

They include some measurements like the nuchal
translucency or fold renal pelvis
and others, as well
as some non measurement markers.

Urinary Tract Dilatation

Urinary tract dilatation is yet another area
in which fetal measurements play an important role.

Here I'll be quoting from recommendations based on a consensus statement from a few years ago, from pretty recently.

It's important to diagnose urinary tract dilatation in utero when it is there
because it's been proven to be able to lead to prevention of renal dysplasia
and renal failure after birth.

If you know about dilatation in utero,
and the key measurement is the AP diameter
of the renal pelvis on a transverse view
of the fetal abdomen here, different measurements
and different degrees of fluid
or amounts of fluid by your eyes,
you'd probably say this is normal,
that's normal, that's normal.

This one probably isn't, and those probably aren't as well.

The specific criteria are as shown here from the
consensus conference between 16 and 28 weeks.

Anything between four and six millimeters is borderline
showing a mild increased risk of postnatal pathology.

Seven to nine millimeters is borderline after 28 weeks.

Anything above those indicates a moderate to high risk
of postnatal pathology.

Below those it's normal.

Also, there's high risk if there are other non measurement
criteria such as peripheral calyceal dilatation,
parenchymal thinning, or abnormality and others.

Ventriculomegaly

Ventriculomegaly is yet another area
where measurements are important.

This is the easiest one.

First of all, we measure it at the standard way, place to measure it is the lateral ventricular width
at the level of the atrium, which is at the level
of the posterior end of the choroid plexus.

Even though this is drooping a little,
it's completely normal.

Six millimeter is normal here, big droop,
and it's 14 millimeters, almost 15 millimeters.

The reason this one's easy to remember is
that there's a single upper limit
of normal from 16 weeks onward, and that's 10 millimeters.

Anything above that is mild or moderate or severe
ventriculomegaly.

Placenta Previa Assessment

Another measurement distance from the placental edge
to the internal cervical os in the assessment of placenta previa, also from the fetal workshop
that I talked about, the recommendation there was,
and I think it's a good one, is the key thing
to look at if you blow up this area,
is the distance from the internal os
to the placental edge.

Simple recommendations don't use terms like partial previas or marginal previas standard terminology.

They recommend,
and I like it is, if the placenta covers the os,
it's a previa if it's within 20 millimeters,
but not over the os it's low lying.

If it's greater than 20 millimeters, there's no previa.

These are examples of no previa, low lying and previa.

Cervical Length

Cervical length is another measurement,
and this is useful in at least some cases in assessing the
risk of preterm delivery.

Best measured on a transvaginal scan
and anything under two and a half centimeters,
25 millimeters is abnormal when there's funneling.

The measurement is from the end of the funnel
to the external os.

There are very specific criteria
that have been put out there for how to measure the cervical length.

I won't go into them item by item,
but it's an important, like all measurements important
to measure accurately.

Fetal Anemia

Fetal anemia.

Yet another area in which measurements can be important.

The measurement
that is most relevant is the peak systolic
velocity in the middle cerebral artery.

When it's more than 1.5 multiples of the median,
it's predictive of fetal anemia,
and that can be confirmed if necessary by doing a fetal blood sampling from the umbilical vein.

How do you know how many multiples
of the median you're dealing with?

Typical measurement is seen here measuring the this is the circle of Willis
and the middle cerebral artery getting the
peak systolic velocity.

What do you do to get the multiples of the median?

The simple way to do it is to go
to this website from perinatology.com,
put in the gestational age
and the peak systolic velocity and outcomes.

Fetal Compromise

The multiples of the median
fetal compromise is yet another area
where measurements apply.

The rationale for doppler
and looking for fetal compromise is that many cases
of fetal compromise are thought to be due
to small vessel disease in the fetal placental and
or uteroplacental circulations.

Elevated indices, and these are other measurement
or number of elevated resistive indices, or other indices,
are in these circulatory systems,
may be useful in the diagnosis of fetal compromise.

An elevated systolic
to diastolic ratio in the umbilical artery is worrisome.

These are the what we consider
to be elevated in our own department.

Anything above four before 30 weeks above three
and a half, at 30 to 34 weeks and above three
after 34 weeks are all worrisome
for fetal compromise.

Absent or reverse diastolic flow are much more worrisome.

Important Routine Measurements

Overall, which measurements are important to do in the first trimester routinely measure
the crown rump length.

The early embryonic heart rate is useful for prognostic purposes.

The nuchal translucency, I have to say,
we do it on a hundred percent of our roughly 10 to 14 week pregnancies.

I put it in different color
because the AIUM et cetera guidelines suggest doing it only
on a subset of patients.

In the second
and third trimester, routinely measure the head, abdomen,
and femur for age, weight, and weight percentile.

The nuchal fold between 16 and 24 weeks and 20 weeks.

I mean, always assess the fetal lateral ventricles,
renal pelvis, and the distance
from the placental edge to the cervical os.

Either measure them routinely
or at least look at them routinely
and measure if there's a questionable
and the amniotic fluid.

You can measure or assess subjectively
and a bunch of different uses of measurements
that you can use, not routinely,
but in specific situations.

Conclusion

I've come to the end of a pretty long list of different uses of measurements.

Numbers can be boring.

I hope that I haven't set you yawning here.

I hope that it's been useful to you.

Thank you for your attention.