Obstetrical Measurements: Which Ones are Important?

Introduction

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

I'll be lecturing today on fetal measurements, focusing on which are the important measurements to use and how to use them in obstetrical ultrasound.

Uses of Obstetrical Ultrasound Measurements

There are a number of uses of obstetrical ultrasound measurements. The two main ones are to determine the gestational age, which is used in order to estimate the due date of the pregnancy, and also to estimate the fetal weight and the weight percentile.

Those are the main ones, but there are many others. And those include diagnosing or contributing to the diagnosis of a number of conditions, early pregnancy failure, fetal growth problems, and others as listed here.

I'll be covering the first two items, determining gestational age and estimating fetal weight and weight percentile. And I'll talk about some, though not all of the other uses listed on the bottom part of the slide.

Guidelines from Professional Societies

Let me address one thing before I get into my own talk. What's in the standard guidelines of a number of societies for measurements in obstetrical ultrasound and in particular, the American Institute of Ultrasound and Medicine, the American College of Radiology and the American College of Obstetrics and Gynecology put out guidelines and they keep them in sync with one another.

So what's in those guidelines? In the first trimester, the guidelines of those societies state that the crown rump length should be recorded when possible, which really means that the crown rump length should be measured and recorded as soon as the embryo is visible, which is at about six weeks.

The mean sac diameter they say may be recorded when the embryo is not identified. And the nuchal translucency should be measured in conjunction with serum biochemistry during a specific age interval for those patients desiring to assess their individual risk of aneuploidy.

These are taken directly from the guidelines of these societies. So they're saying in this one that the nuchal translucency is to be measured two important things. One is it should be measured in conjunction with serum biochemistry, maternal serum blood tests, in other words, and that it should be used in those women who wish to know their individual risk of aneuploidy. That's what the guidelines say.

In the second and third trimester, the guidelines talk about assessing amniotic fluid, either qualitatively just looking and determining whether the amniotic fluid volume is appropriate, high or low, or semi quantitatively, the amniotic fluid index, the deepest pocket or a two diameter pocket.

The guidelines say with respect to gestational age, they're pretty broad. They say you can use a variety of parameters. The important thing that they state, and this is a very important one, and we'll talk about it a little bit shortly, is not to redate a pregnancy if an accurate prior scan has been done.

Fetal weight estimation is talked about in the guidelines. Again, they're not specific, they don't say what measurements, but they do say that you can use a variety of measurements. They don't tell you what table or formula to use, and they suggest comparing the fetal weight estimate to published guidelines.

The nuchal fold may be helpful during a specific age interval according to the guidelines.

Determining Gestational Age

Let's get into specific uses, first determining gestational age or estimating the due date. I'll take information from a 2014 committee opinion from the American College of Obstetricians and Gynecologists, or ACOG, AIUM, as well as the Society for Fetal Medicine, their committee opinion on methods for assessing the due date.

Terminology for Gestational Age

First, a little bit of terminology with respect to gestational age. First the concept of conceptual age. That's the time since conception. And that is actually the age of the pregnancy. If conception was 10 weeks ago, then the pregnancy has been around for 10 weeks.

Another term that's used is menstrual age, which is the time since the first day of the last menstrual period, since conception occurs approximately two weeks after the first day of the last period. Menstrual age is approximately conceptual age plus two weeks.

And a third term is gestational age. And gestational age is a measure of the age of pregnancy that approximates menstrual age, but it's determined using potentially one or more of a number of factors.

So one is in women in whom the date of conception is known. The gestational age is the conceptual age plus two weeks. That's the most accurate estimate of the gestational age.

Gestational age is sometimes taken as synonymous with the menstrual age, the time since the first day of the last menstrual period. The gestational age, in other cases is used or is determined based on ultrasound measurements and in women who are pregnant via assisted reproductive technologies, such as in vitro fertilization.

Gestational age can be derived using the information from the IVF or other assisted reproductive technique. So the gestational age is the one that's most used. It's roughly equal to the menstrual age, but it can be determined by a number of different factors depending on the patient's individual situation.

Importance of Accurate Gestational Age Assessment

Why is accurate assessment of gestational age important? It has a number of uses, and those include assistance in making obstetrical management decisions, such as the time to deliver the pregnancy if the patient seems to be going post dates.

Measurements, accurate assessment of gestational age is useful and important when we're assessing fetal growth, to know how big the fetus should be or how big various parts should be, such as the length of bones or the width of the head. We need to compare those to gestational age. So we need to have an accurate assessment of gestational age.

Knowing the gestational age is important for scheduling and interpreting certain prenatal tests, such as maternal blood tests or the nuchal translucency measurements.

And finally, it's useful for interventions to avoid preterm and post-term births.

Methods for Assigning Gestational Age

Gestational age can be assigned by the last menstrual period that's easy and quick, but there are some sources of inaccuracy, and those include variable and irregular cycle length. If a woman has a menstrual cycle that varies from month to month or is not the usual 28 days, that can give inaccuracy in estimation of the age of their pregnancy based on the LMP.

There can be variable time from the first day of the last period to ovulation and implantation. An important limitation or source of inaccuracy of dating by the LMP is misinterpretation of vaginal bleeding that's not related to a menstrual period as being a period, for example, bleeding in early pregnancy.

And finally, a lot of women have poor recollection of their last menstrual period, and there is plenty of literature to support the fact of poor recollection.

Another method of assigning gestational age in the first trimester is by ultrasound. And there are a couple of ways to do it. If the ultrasound finding is a gestational sac with no identifiable embryo, and that's at about five to six weeks of pregnancy, that's what we see on ultrasound.

We can assign gestational age via either the mean sac diameter or the contents of the gestational sac. We actually, in our own department, use the sac contents to assign gestational age based on ultrasound.

If we see a small gestational sac with no yolk sac and no embryo, we say the findings are consistent with a five week pregnancy. If we see a gestational sac and a yolk sac, but no embryo, we say that the findings are consistent with five and a half weeks. So that's how we date the pregnancy.

But the mean sac diameter can also be used to date early pregnancies prior to identification of an embryo.

Once, if the ultrasound finding is a gestational sac and an embryo, as it is between about six and 14 weeks of pregnancy, we assign gestational age via the crown rump length. And with either of these ultrasound methods, we have an accuracy of plus or minus 0.5 to 0.7 weeks, or an accuracy of about plus or minus three to five days.

So, ultrasound to assign gestational age in early pregnancy is very accurate, plus or minus three to five days is highly accurate for assigning gestational age.

Here is an example of gestational age assignment by ultrasound at the five to six week period. Here we see sagittal and transverse views of the gestational sac. We have taken three measurements. As you can see here, we have two measurements, 0.8 three centimeters, or 8.3 millimeters, a half a centimeter, or five millimeters, seven and a half millimeters, and we can average those into a mean sac diameter of 6.9 and look it up in a table and say, aha. The mean sac diameter is 6.9 millimeters. That corresponds to a gestational age of 5.6 weeks.

Alternatively, again, we can date by sac contents here. The sac contents are a gestational sac with no yolk or embryo. And based on the table on the prior slide, that corresponds to a gestational age of five weeks.

Another example of gestational age assignment by ultrasound. The early first trimester here, if we take the mean sac diameter, the average of these three numbers, it's 9.9, and if we look it up in a standard table that corresponds to a gestational age of six weeks, if we date by the sac contents, we say there's a gestational sac, there's a yolk sac, but no embryo, we would date that as five and a half weeks.

So, dating by mean sac diameter. We date the pregnancy at six weeks, dating by sac contents at five and a half weeks.

And then once we see the embryo in the first trimester, as we do here in this early pregnancy, and we see the embryo from six weeks onward, we would date the pregnancy based on the crown rump length. The crown rump length is 5.6 millimeters, and that corresponds to gestational age of 6.4 weeks.

How do we assign gestational age in the second and third trimesters based on ultrasound findings? So, if the biparietal diameter and the occipital frontal diameter are both measurable, we should assign, we assign gestational age via head measurements that account for head shape, either the head circumference or the corrected biparietal diameter.

And if the biparietal diameter is measurable, in other words, you can measure the width of the head, but the length isn't accurately measurable, which occurs in some cases, we can assign gestational age either by the biparietal diameter or the femur length.

And using these, the accuracy progressively worsens from about plus or minus 1.2 weeks in the early second trimester to plus or minus three weeks in the third trimester. And those are for the head circumference or the corrected biparietal diameter. They're slightly worse for the biparietal diameter alone or the femur length.

The abdominal diameter or circumference is not reliable for determining gestational age.

And I talked about the corrected biparietal diameter. It's one of the two head measurements that account for head shape. So if we have here a head where if this is the fetal head, we measure the biparietal diameter as the width of the head from leading edge to leading edge and the occipital frontal diameter from mid skull to mid skull.

We can either take a head circumference measurement and that accounts for the shape of the head, not just the width of the head or the corrected biparietal diameter, which is what it refers to is correcting the biparietal diameter for the length of the head.

If a head has standard or average shape, there's no difference between the corrected biparietal diameter and the uncorrected biparietal diameter. If you have a longer dolichocephalic head the corrected BPD is somewhat longer than the BPD, and if you have a short or brachycephalic head, the corrected BPD is less than the uncorrected BPD.

So either the head circumference or corrected biparietal diameter take into account the shape of the head, and here is a fetal head. It's about standard shape. The head circumference is about 200 and 67 millimeters. If you can faintly see the dotted line around the head.

If we measure the biparietal diameter and the occipital frontal diameter, we can get a corrected BPD of 69.4 millimeters. It's slightly bigger in this case, than the biparietal diameter of 67.8 millimeters, but not much different.

If we have a dolichocephalic head here. The measurement or we would use in a case like this, the we would base the gestational age on either the head circumference or the corrected biparietal diameter.

If we can only measure the width of the head, as in this case, and we don't get a reliable measurement of the length of the head, in other words, only the width is measurable, we would base the gestational age on the biparietal diameter or the fetal or the femur length, with numbers such as here.

Recommendations for Assigning Gestational Age

Some of the recommendations based on the consensus document that I showed you earlier for assigning gestational age are as follows, at the time of the initial ultrasound at or beyond six weeks based the gestational age on the last menstrual period, unless the woman is unsure of her LMP or the LMP is inconsistent with what we see on ultrasound. And this is when we would use the ultrasound dating instead of the LMP.

If the GA by LMP is less than nine weeks, we use ultrasound dating only if the difference between the ultrasound date and the ultrasound date is more than five days. So if, for example, the GA by LMP is nine weeks, and the GA by ultrasound by the crown rump length is 10 weeks, the difference is more than five days. And therefore, we would use ultrasound dating if the GA by LMP is between nine and 16 weeks, we use the ultrasound dating only if it's more than seven days discrepant and so on.

You can see that by the late third trimester, or by the third trimester from 28 weeks onward. We would only use the ultrasound dating if the ultrasound date is more than three weeks discrepant from the LMP date.

And at after the initial ultrasound, we would redate only in very rare circumstances. So if a woman has had a ultrasound at, say, 10 weeks, and then she has a second ultrasound anytime thereafter, we would use the age that we determined at 10 weeks and add the number of intervening weeks we wouldn't.

Redate in pregnancies achieved via assisted reproductive technologies such as IVF, we would use dating derived from that technique. For example, use the embryo age and the transfer date in IVF patients.

Estimating Fetal Weight and Weight Percentile

Okay, well, let's go on from determining gestational age to estimating fetal weight and weight percentile. Routinely we measure, we estimate the fetal weight via measurements of the head, femur, and abdomen.

And once we have an estimate of the fetal weight and an assignment of gestational age, we assess the fetal size in relation to the age via the estimated weight percentile.

And if the estimated weight is less than the 10th percentile, we suspect growth restriction. And then we would undertake methods to attempt to determine the cause of the small fetal size and to monitor the pregnancy closely.

Discussion of growth restriction is a major topic unto itself, and I won't get into the details here only to say that we suspected when the estimated fetal weight is less than the 10th percentile for gestational age, we use weight tables or weight charts to estimate the weight percentile based on the estimated weight and the gestational age.

Other Uses of Fetal Measurements

Failed Intrauterine Pregnancy

Okay, well, let's get onto some of the other uses of fetal measurements. It's certainly used for more than determining gestational age and estimating the fetal weight and the weight percentile. And we'll talk about some of these.

The diagnostic criteria for the failed intrauterine pregnancy or miscarriage use fetal measurements such as a crown rump length of at least seven millimeters and no heartbeat indicates definite pregnancy failure. A crown rump length less than seven millimeters, and no heartbeat is suspicious, but not definitive for pregnancy failure.

And some other measurements such as a mean sac diameter of at least 25 millimeters and no embryo is indicative of definite pregnancy failure.

So this is a way that measurements are used to diagnose a problem in early pregnancy, namely failed intrauterine pregnancy or miscarriage.

Amniotic Fluid Assessment: Polyhydramnios and Oligohydramnios

Let's move on to another place where fetal measurements or amniotic measurements can be used in pregnancy. And that's in the diagnosis of polyhydramnios and oligohydramnios. And here I will refer to a consensus document on fetal imaging that was put together in 2014 by a number of societies including obstetrical societies and radiologic societies and others.

So we will be discussing ultrasound diagnosis of polyhydramnios and oligohydramnios.

The most accurate way to know the amniotic fluid volume is by a technique known as dye dilution, where you inject concentrated dye into the amniotic fluid, let it mix evenly and then remove it a few minutes later and see how much it's been diluted. That can give you accurate determination of the amniotic fluid volume in milliliters or any other measurement of fluid volume. And that has been done.

And you can see here a table of the median amniotic fluid volume measured in milliliters at various gestational ages. You can see that it progressively increases until the late third trimester, and then it begins to tail off at and beyond term.

So the most accurate way to diagnose oligo and polyhydramnios would be to use dye dilution on your patient and see if the fluid is high or low compared to where it should be for gestational age. That's obviously impractical and not something that we do.

So we use other methods to assess the amniotic fluid volume by ultrasound. And these are semi-quantitative. The fully quantitative would be the dye dilution method.

Subjective assessment is one way. And the subjective assessment simply refers to the approach where we scan through the pregnancy through the gestational sac, and make a hopefully educated assessment of whether the fluid is normal, high or low for what we expect to see at the patient's gestational age.

And then there are some semi-quantitative methods, including the single deepest pocket measurements there. The usual criteria are that if the single deepest pocket is less than one or two centimeters, that constitutes oligohydramnios. If the single deepest pocket is greater than eight centimeters, that constitutes polyhydramnios.

Another semi-quantitative method is the amniotic fluid index, which is the sum of the four deepest pockets in four quadrants of the gravid uterus quadrants determined by horizontal and transverse lines through the woman's umbilicus.

And if the amniotic fluid index, the sum of those four deepest pockets is less than five centimeters that constitutes oligohydramnios greater than 18 to 20 centimeters polyhydramnios.

So you can see there are really three main ways that we can assess amniotic fluid volume by ultrasound. Again, subjective assessment, single deepest pocket or amniotic fluid index.

Here on this video clip, we make a subjective assessment that in this 19 week fetus that the amniotic fluid volume is normal. On the right we have an amniotic fluid index measuring four deepest pockets in four quadrants, adding them together to get an AFI of 12.4, which is normal.

So both subjective assessment and amniotic fluid index reach a conclusion that the fluid is normal here.

Subjective assessment looks like the fluid is somewhat high at 30 weeks, and the amniotic fluid index confirms that at 37.5.

So, which one of these should we use? Well, let's look at the pluses and minuses of each subjective assessment is quick and efficient. It allows us to account for gestational age variation in amniotic fluid volume, at least subjectively, we have a pretty good idea if we're experienced at knowing how much fluid you expect to see at 15 weeks, 25 weeks, 35 weeks. And we account for that when we make our subjective assessment.

But because it requires experience to know how much fluid to expect to see subjective assessment may be unreliable with an inexperienced operator, it is best documented using a video clip as I showed you in the prior two slides.

The semi-quantitative amniotic fluid measurements, including the deepest pocket and amniotic fluid index. They are simple and quick, but they do have some downsides. Even with oligohydramnios, you can get one or more pretty deep pockets that don't have a lot of fluid because they are very thin and narrow found in crevices such as the neck or between the legs.

So you can get a deep pocket, even with oligohydramnios in some cases, the amniotic fluid index may be affected by fetal movement, and they're really not mathematically valid. Linear measurement like a deepest pocket really doesn't in any direct way correlate with volume.

And to illustrate that here are two diagrams of two people. They're linear height, they're a linear measurement of them is the same, they're the same height, but they're very different volumes.

So really, assessing amniotic fluid via one or even four linear measurements is not a mathematically valid or that accurate, a method of assessing volume.

So all of these methods have pluses and minuses, which one's most accurate. There are a couple of published studies that used dye dilution as the gold standard and asked which of these is the most accurate. And what the studies have found is there's really no significant difference between subjective assessment and the amniotic fluid index. The same would likely be true with the single deepest pocket measurement.

So there's nothing really, no big reason to choose among them, on the downside. Neither of those is especially accurate at diagnosing increased or decreased volume compared to the gold standard of dye dilution.

So what's the recommendation of the fetal imaging workshop that whose paper or whose front page I showed you a few slides back? Their recommendation is that subjective assessment and semi-quantitative methods are both acceptable. And the single deepest pocket appears to be preferred over the amniotic fluid index because in one or more studies, the single deepest pocket leads to fewer interventions than the AFI without any significant difference in outcome.

Urinary Tract Dilatation

To go to another kind of abnormality where fetal measurements help us in making diagnoses. And that's assessment of urinary tract dilatation. And what I'm going to talk about here briefly is the Society for Fetal Urology Consensus Statement on the evaluation and management of antenatal hydronephrosis.

Early detection. Diagnosing fetal urinary tract dilatation is very important because early detection of urinary tract dilatation in utero can lead to either prenatal or postnatal interventions that prevent renal dysplasia and renal failure.

The key measurement for diagnosing urinary tract dilatation or the absence of it is the anteroposterior or AP diameter of the renal pelvis on a transverse view through the fetal abdomen as in these cases.

Here we have pretty small amount of fluid in the renal pelvis looks pretty normal. The measurements are both about four millimeters. Here we have one that looks a little big and the other looks pretty normal. And here we have both looking pretty abnormal.

But what criteria, what quantitative criteria can we use to determine whether these measurements each of them is normal or abnormal? According to the consensus conference that I referred to a couple of slides back, whether a AP measurement of the fetal renal pelvis is normal or not, depends on the gestational age between 16 and 24 weeks, anything under four millimeters is considered to be normal.

At 28 weeks and beyond, anything under seven millimeters of the AP diameter of the renal pelvis is considered to be normal. Anything above seven millimeters between 16 and 28 weeks is abnormal, indicating a moderate to high risk of significant problems after birth.

Whereas at least 28 weeks from 28 weeks onward, anything 10 millimeters and beyond is abnormal. And the intermediate range indicates is borderline hydronephrosis indicating a mild increased risk.

There are other high risk criteria besides the AP diameter, and those include if you see peripheral calyceal dilatation, renal parenchymal thinning or abnormality. Dilated ureter or distended bladder are also factors to take into account when determining if there's urinary tract dilatation.

Ventricular Megalaly (Hydrocephalus)

Okay, switching gears. Another abnormality that we diagnose or rule out based using fetal measurements is ventricular megalaly of the lateral ventricles of the fetal brain, also known as hydrocephalus.

And the key measurement there is the width of the lateral ventricle at the level of the atrium. The atrium of the ventricle is right at the posterior aspect of the choroid plexus. Here, you can see it slightly drooping, which is fine. And right at the end of it is where we measure the ventricular width.

Here you, in the first one, you can see it's measuring 5.8 millimeters, and the second one looks big and it's measuring 14.6 millimeters.

So what's normal, the answer is pretty straightforward. Normal is anything up to 10 millimeters. 11 to 12 millimeters is mild ventricular megalaly, and anything 13 or above is moderate to severe ventricular megalaly.

Placenta Previa

Yet another use of fetal measurements is diagnosing placenta previa. And here the key measurement is the distance from the edge of the placenta to the internal cervical os. As you see here, it's measuring 1.56 centimeters or 15.6 millimeters.

What's normal and what's abnormal? The fetal imaging workshop recommended the following, pretty simple terminology and diagnostic criteria. And the key measurement is measuring from the edge of the placenta to the internal os.

If that is greater than 20 millimeters, no previa. If the placenta covers the os, it's a previa. And if it's less than 20 millimeters, but not covering the os, we call it low lying.

Terminology such as marginal and partial previa were suggested that we should it was suggested the fetal imaging workshop, that those terms be avoided and that everything be called when we do an ultrasound, either no previa, low lying, or a placenta previa.

And here are examples on these transvaginal scans here. The measurement from the edge of the placenta to the internal os is three centimeters, no previa. Here it is one and a half centimeters, 15 millimeters, it's between zero and 20. It's low lying. And here the placenta completely covers the os, which is here. It's a placenta previa.

Cervical Length

Yet another measurement is the cervical length. And the use of the cervical length in pregnancy is to assess the risk of preterm delivery and what we're doing when we measure the cervical length, as we'll see on some examples, if there is no opening in there. Internally we measure from the internal os to the external os.

And if there is opening or funneling internally, we measure from the edge of the funnel to the external os. So we're measuring the length of the closed cervix running down the middle, and we use it again to assess the risk of preterm delivery.

Here is a nice normal cervix. Measuring over three centimeters here is somewhat short or borderline at 2.2 centimeters. And here we have funneling and shortening measuring 11.8 millimeters or about 12 millimeters.

Very specific measurement criteria have been put forth to measure cervical length by the CLEAR group, the cervical length education and review group. And they recommend measuring transvaginally, expanding the image. So the cervical os occupies most of the image having a empty maternal bladder and other criteria.

So be familiar with the criteria and use the criteria if you're gonna measure cervical length. Now, they again, they recommend transvaginal ultrasound.

Here's a problem with there are a few problems with transabdominal ultrasound here. We look at the cervix and we really can't see it very well, 'cause the head is shadowing it. In other cases, a large bladder may mess up our measurement of cervical length.

So is the cervix normal transabdominal? We really can't tell 'cause we're not seeing it very well. We go on to do a transvaginal scan, and you can see that the cervix is entirely open or funneled, and there is virtually no cervical length left.

So transabdominal is not great, transvaginal as much better and should be used. But there is an important potential pitfall that you should avoid. And that's if you push too hard on with the transvaginal transducer on the cervix, you can take an open cervix and close it.

As you're seeing here on the left, you can see the video clip as I'm pushing in and closing. What was an open cervix? You can see the correct measurement is 15 and a half millimeters as seen up here as funneling. But the important thing is 15 and a half millimeters. When we close it, by pressing, it's actually measuring three centimeters.

So you have to be very careful not to press too hard. Transvaginal scanning is good, but again, it does have that pitfall.

And another pitfall of transabdominal or transvaginal scan is that a lower segment uterine contraction can falsely elongate the measurement that you get of the cervix here. We're measuring from what looks like the internal to the external os and getting a measurement over five centimeters.

But beware because you can see this rounded or bunched up configuration internally. Beware cervical contraction if you think there may be a contraction. Wait a few minutes here. After the contraction resolved, same patient. It's actually measuring 24 and a half millimeters.

Non-Invasive Assessment for Fetal Anemia

Yet another use of fetal measurements is in the assessment, non-invasive assessment for fetal anemia. And here one of the key articles was in the year 2000 by Jean Carla Mari and others on non-invasive in the New England Journal of Medicine, a non-invasive diagnosis of fetal anemia by doppler ultrasound.

The key finding that comes out of that paper in that study is that a peak systolic velocity in the middle cerebral artery that's greater than 1.5 multiples of the median for gestational age is predictive of fetal anemia.

So what we're doing is measuring as we see here, this is the circle of Willis, the middle cerebral artery we're measuring. We're getting a doppler signal and we're measuring the peak velocity or the peak systolic velocity in the middle cerebral artery.

And this is what we would use to compare it to determine the number of multiples of the median for gestational age. There's an online calculator that you can use, here, this is the web address.

And using that calculator, you would put in a gestational age, the middle cerebral peak systolic velocity, and hit calculate. And it will give you the median, peak systolic velocity for this age is, and it'll tell you, and it tells you the multiples of the median.

And if it's greater than 1.5, you have to be worried about fetal anemia and consider management or diagnostic tests that are relevant when fetal anemia is concerned.

Summary of Important Measurements

Just to sum up which measurements are important in the first trimester, we routinely at our own institution, we routinely measure the crown rump length. We also measure the early embryonic heart rate because slow heart rates can be indicative of subsequent fetal demise. If it's slow, nuchal translucency can be measured again, in some circumstances best done in collaboration in women who are also who wanna know about the possibility or their risk of fetal aneuploidy and who are planning to have appropriate serum blood tests.

In the second and third trimester. What's important are measurements of the head, abdomen, and femur to assign gestational age at the time of the initial scan to estimate fetal weight in the third trimester, and to determine the weight percentile.

Measuring the nuchal fold is important at 16 to 20 weeks. Measuring or assessing cerebral lateral ventricles, renal pelvis and the distance from the placental edge to the cervical os are key. And the amniotic fluid volume should be measured or assessed subjectively.

And then there's some other measurements that we use in specific circumstance. If we see a markedly shorter deformed femur, we'll measure all the long bones. We're doing a targeted aneuploidy scan. Measurements of the nasal bone are humerus can be valuable in women at high risk for preterm delivery, cervical length is valuable and others.

So I hope this survey has been useful to tell you how measurements can be used and which should be used on a routine basis or in specific situations during pregnancy. Thank you for your attention.