There is likely no more important piece of information used to manage pregnancies than the correct assessment of gestational age. The accurate determination of gestational age underlies all obstetric management decisions, from the recommendation of the correct gestational age appropriate screening or diagnostic genetic tests to the timing of diabetic screening, to the administration of anti-D immunoglobulin, to group B streptococcal testing, and most importantly, to the assessment of fetal viability and well-being and the appropriate timing of delivery.

Before the development of ultrasound technology, the menstrual age was based on the reported first day of the patient’s last menstrual period (LMP) and confirmed by physical examination of the gravid uterus and postnatal examination of the fetus. The use of the patient’s LMP to establish dating is fraught with imprecision, particularly if the woman does not have a typical 28- to 30-day menstrual cycle, has had irregular menses resulting from lactation or oligoovulation, or became pregnant while on birth control. A woman’s memory of her LMP may also be imprecise, with studies reporting up to 45% of women being unsure of their LMP date.¹ Even with a certain LMP, the ovulation to implantation interval may vary by as many as 11 days and tends to occur later than calculated.² Further complicating identification of the exact timing of fertilization, sperm can survive in the reproductive tract for 5 to 7 days. Understanding the inherent variations from the “normal” menstrual cycle (Table 6-1) explain why the LMP has been shown to consistently underestimate gestational length and potentially lead to nonindicated obstetrical procedures such as induction of labor for postdates.³

Based on these factors, it has been shown that the gestational age can be more accurately established with sonographic parameters in the first or early second trimester when genetic variances in each pregnancy have minimal impact and the fetus grows at a relatively standard rate.⁴

Historically, the assessments of fetal dimensions were measured using static radiological techniques. The development of continuous mode ultrasound made it possible to safely measure the bones and soft tissue structures of the fetus both more rapidly and reliably than with x-rays. The use of such measurements now allows the following questions to be answered:

1. 
   

   What is the estimated gestational age of the fetus?

   
   
2. 
   

   Is the fetus of appropriate size for its estimated gestational age?

   
   
3. 
   

   Are there any malformations?

The first question (gestational age), as stated previously, is crucial in the modern practice of obstetrics and is one of the most frequent reasons for referral in countries where routine scanning is not the practice. The second question, the evaluation of fetal growth for the detection of intrauterine growth restriction, is also a major concern because the growth-restricted fetus is at a higher risk for morbidity and mortality.^5-7 Lastly, although decreased access to prenatal care still prevents the accurate dating of numerous pregnancies, with the widespread implementation of ultrasound technology the detection of congenital malformations has become as important as the establishment of the correct gestational age. The presence of fetal anomalies may affect fetal measurements, and the early and accurate identification of major fetal anomalies makes all therapeutic modalities available and allows patients to consider all options for their pregnancy.

The use of ultrasound dating has improved clinical care of pregnant patients. Ultrasound dating has reduced unnecessary postdates induction of labor, even in cases of “certain LMP,” by up to 70%.^8-10 Additionally, the increased accuracy of dating has improved the performance of prenatal genetic screening tests.^9,11,12

To ensure a standardized approach to fetal anatomical screening, the American Institute of Ultrasound Medicine (AlUM), the American College of Obstetrics and Gynecology (ACOG), the Society of Radiologists (SRU), and the American College of Radiology (ACR) have published guidelines describing the recommended structures and the views of each that should be evaluated during routine midtrimester (18-20 weeks’ gestation) ultrasound evaluation (Table 6-2).

In addition, careful and meticulous tables that correlate biometric measurements with gestational age have been developed for almost all fetal biometric parameters and are useful in accomplishing the aforementioned tasks.

In this chapter, we will first review the individual biometric parameters and how they are used to establish estimated gestational age (EGA) and estimated fetal weight (EFW) and then discuss some new concepts in fetal biometry that may improve the accurate identification of fetuses at risk for growth abnormalities and suboptimal neonatal outcomes.

How Are Normal Values Derived?

Although virtually all modern-day ultrasound machines come with multiple preset biometric equations that automatically compute the EGA and EFW based on the measurements included in any single examination, it is still important to understand how normative values for biometry are established. The normal values for a given parameter at different gestational ages are defined by measuring that parameter in normal fetuses of healthy patients with well-established gestational dating. Data collection for this purpose can be either cross-sectional or longitudinal. How normative biometric values are obtained and the differences between the two methods of collection are explained in the following section.

Selection of Patients

For the purpose of establishing biometry norms, the patient population with the best known dates are those who have undergone in vitro fertilization. Similarly, women who undergo ovulation induction and intrauterine insemination have a small error of EGA. Finally, patients satisfying the clinical combination of a regular menstrual history with a known stated LMP confirmed by an early ultrasound (<14 weeks’ gestation) are appropriate to use in establishing biometry norms. Outside of dating criteria that may impact the accuracy of the estimated gestational age, patients with a history of medical, surgical, or obstetric complications that may affect fetal growth rates, and consequently EGA and EFW, should not be included when establishing population standards. Optimally, infants included in any study of biometry should also be followed longitudinally to ensure term deliveries between 37 and 42 weeks with no evidence of growth restriction or congenital anomalies.

Types of Studies: Cross-Sectional or Longitudinal?

As stated, fetal growth may be analyzed by 2 different kinds of studies: cross-sectional or longitudinal. In a cross-sectional study, subjects are examined only once during gestation. In a longitudinal study, individual subjects undergo serial examinations over time.

The advantages of cross-sectional studies are that (1) they can be performed in a relatively short period of time, (2) the data are easier to collect because the patients are scanned only once, and (3) the statistical analysis is easier. They also have important drawbacks in that (1) they fail to characterize individual growth (they describe “population” growth), (2) the stability of the statistical analysis that can be performed on cross-sectional data is not always as good as that from longitudinal studies, (3) they are extremely susceptible to inclusion of fetuses with abnormal growth patterns, poorly established gestational ages, or both, and (4) the follow-up of newborns is complicated by the large number of cases.

Longitudinal studies also have several advantages: (1) the gestational age has to be defined in fewer patients, (2) gestational age is established in early pregnancy, (3) abnormal growth curves are easily recognized, and (4) the statistical analysis allows stronger curve fitting and computation of velocity curves (Table 6-3). Unfortunately, by their design, these studies require that the same fetus be scanned throughout gestation. This considerably increases the time needed to collect data and calls for high maternal motivation. There are many mathematical, biological, and epidemiological arguments to favor results obtained by longitudinal versus cross-sectional studies. The discussion of these is beyond the scope of this chapter.

How to Compute the Confidence Limits

Besides being able to describe the mean growth, one must be able to answer the question: How far from the mean can an individual be and still be considered normal? Or, in statistical terms: What is the dispersion around the mean? The statistical parameter that measures this dispersion is the standard deviation. The smaller the standard deviation, the less variability is present of the sample around the mean. The standard deviation is also used to define the statistical limits of normality. These intervals are called confidence limits (Figures 6-1 and 6-2). Traditionally, the confidence limits are set at the 5th and 95th percentiles (±1.66 standard deviations) or at the 1st and 99th percentiles (±2.38 standard deviations). Two standard deviations correspond to 95% of the population, 2.5% being below the normal limits and 2.5% being above. Outside the lowest and highest percentiles, the parameter is considered statistically abnormal. With the commonly used 5th and 95th percentiles, it is important to remember that 10% of patients tested will be outside normal limits. This does not mean, however, that the patient is necessarily abnormal.

This concept becomes clinically relevant with regards to biometry when trying to define the growth-restricted fetus. By definition any fetus with an EFW below the 10th centile is labeled as “growth restricted” and subjected to increased antenatal testing with recommended delivery by early term (37 weeks). A majority of infants that fall below the 10th centile, however, will demonstrate normal growth curves and will either be simply constitutionally small fetuses, have poor dating, or have had their EFW underestimated secondary to the inherent inexactness of 2D imaging in the prediction of EFW. The most clinically relevant definition of what measurement actually should label an infant as “growth restricted” has been challenged in the recent literature. The PORTO trial prospectively evaluated 1100 consecutive gestations with an EFW below the 10th centile. The trial demonstrated that abnormal umbilical artery Dopplers and an EFW below the 3rd centile were most consistently associated with adverse perinatal outcomes, suggesting that the definition of IUGR for all infants below the 10th centile may need amending.¹³ As demonstrated by this clinical example, beyond simple statistical terminology, biometric cut-offs need to balance the sensitivity and specificity of the test and take into account the clinical implications associated with each standard.

Application of Growth Curves

There are dozens of equations to calculate an estimated gestational age and EFW. Most ultrasound units use set formulas preinstalled in their machines to maintain consistency. It is important for the individual practitioner to be aware of the equations programmed into their specific machine and understand in what clinical situations there may be increased variability in the predicted gestational age or fetal weight. The decision of which equation to use for one’s ultrasound practice is an important one, and the following factors are important to consider. (1) The equation should be derived from a study based on a large sample of data covering a wide range of values for the independent variable. (2) It should adequately represent the extremes of the curve. (3) If there are many equations that fulfill these criteria, the curve selected should be based on longitudinal rather than on cross-sectional studies. (4) The study that the curve is based on should span the range of gestational age being covered. For example, some studies describe the growth of the femur from 12 to 22 weeks; using these studies outside those limits would be inaccurate. It is beyond the scope of this chapter to discuss all the curves that have been published. The guidelines should help the reader to decide which curves are correct.

Definition

As stated in the “definitions” section at the beginning of this chapter, the EGA is, for clinical purposes, synonymous with the menstrual age and is the standard terminology used in obstetric practice for evaluating the length of the gestation. The weeks are always counted as completed weeks and current days of the next week. For example, a patient whose LMP started on January 1 will be 4 weeks and 3 days on February 1.

Parameters Used for Gestational Age Assessment

The best biometric parameter to use when dating a gestation is dependent on the gestational age at the time of the first ultrasound examination. As will be discussed, single biometric parameters are easier to use and more accurate to predict EGA in early gestation, with multiple parameters becoming easier to obtain and becoming more accurate for EGA prediction as the gestation progresses. A grounding premise in pregnancy dating is that as biologic variation in the gestation becomes wider with advancing gestation, the accuracy of any single parameter or group of parameters to estimate the EGA decreases.⁴ As a result, once the dating of a pregnancy is established with a reasonably well-performed ultrasound, by convention we do not change the dates with later examinations. In all the tables in this section, gestational age is expressed in weeks and days. The parameters proposed to establish gestational age are detailed in the following section.

Gestational Sac

The structures visualized in early gestation appear in a sequential fashion and at set time intervals and are valuable in determining not only the estimated gestational age, but also the viability of the pregnancy. The order of structural appearance is (1) gestational sac, (2) yolk sac, (3) fetal pole, and (4) fetal heart activity. In early gestation transvaginal ultrasound can demonstrate the gestational sac and yolk sac better than transabdominal scanning and is the preferred method for the identification of these structures.¹⁴ Once a fetal pole is identified, however, either transvaginal or transabdominal scanning can be used for dating with equal accuracy.^9,15-17

The gestational sac is the first structure to appear and can be first seen when it reaches a mean sac diameter (MSD) of 2 to 3 mm, which correlates with an estimated gestational age of 4 weeks and 3 to 4 days (32-33 days).^18,19 The MSD is obtained by taking three measurements in orthogonal planes and then calculating the average diameter. The MSD can then be compared to gestational age tables to correlate with an EGA.²⁰ The MSD increases roughly 1 mm a day in early gestation,^21-23 and although its reliability for prediction of EGA decreases with advancing pregnancy, it remains fairly precise up to a MSD of 14 mm²⁴ contingent on the technical accuracy of the measurements. In other words, nonorthogonal planes, or nonmaximal measurements, may affect the dating calculation.

The yolk sac is an intermediary structure that is seen after the appearance of the gestational sac and prior to the appearance of the embryo. It can be visualized by transvaginal scanning when the MSD is 6 to 12 mm.²⁵ Although not useful in determining EGA, its timely appearance is reassuring to the patient and practitioner that the early gestation is developing as expected.²³

Alterations in the pattern of appearance of the aforementioned structures may be suggestive of a nonviable or failed gestation and also must raise the concern for an ectopic pregnancy if the ultrasound findings do not correlate with hCG levels. An early gestational sac must also be differentiated from fluid within the endometrial cavity, the so-called “pseudogestational sac,” that may be present with ectopic pregnancies. Table 6-4 compares the characteristics of a true gestational sac versus a pseudosac. Furthermore, to prevent the misidentification of an early gestation as nonviable, guidelines have been established to help the clinician in determining the viability of an early gestation based on the measurements and timely identification of the gestational sac, yolk sac, fetal pole, and fetal heart activity (Table 6-5).

Crown-Rump Length

The fetal pole, or embryo, can be consistently visualized by transvaginal scanning at a size of 5 mm, but can be detected when as small as 2 mm. The crown-rump length (CRL) has lower interobserver variability than measurements of MSD,²⁶ and once identifiable, the CRL should be used to determine EGA over MSD.^9,24,27,28

The CRL (Table A6-1) is measured by obtaining the longest demonstrable straight-line length of the embryo or fetus. To obtain the most accurate CRL, the fetus should ideally be imaged in the midline, sagittal plane with the calipers placed at the outer edge of the cephalic pole and the outer edge of the fetal rump (Figure 6-3). Because the CRL can be affected by flexion or extension of the fetus, at least 3 measurements should be obtained and averaged to determine the final CRL result. Care should be taken to avoid the common error of including the fetal limbs or yolk sac in the CRL measurement.

The reason for the high accuracy of the CRL is the excellent correlation between length and age in early pregnancy when growth is rapid and minimally affected by genetics or pathological disorders. The accuracy of the CRL has been confirmed in IVF pregnancies.^9,29-31

As the gestation progresses, however, the CRL measurement becomes less accurate when determining EGA. This is due to both the increased curvature of the fetus and the increased impact of biologic variation that the genetic makeup of the fetus starts to exert on the growth frequency. The CRL has been shown to be predictive of menstrual age with an error of 3 to 5 days (±2SD)^32,33 from 7 to 10 weeks with its narrowest CI when between 7 and 60 mm.^{9,28,29,34,35} After the CRL reaches 84 mm it should not be used to estimate EGA.^9,36

Hadlock evaluated the accuracy of CRL measurements in 416 women³⁵ and determined that the variability in CRL measurements remained constant at ±8% in fetuses measuring from 2 mm to 12 cm and felt that this decrease in accuracy was the result of increasing biologic variability as pregnancies progress. Inherent in this finding is that as the gestation increases so does the inaccuracy in the measurement as a determinant of the gestational age. More recently, Pexsters et al evaluated the CRL in over 3500 pregnancies and noted statistically significant differences in their CRL curves as compared to Hadlock’s CRL curves in the estimation of EGA.³⁷ Compared to their data, at 6 weeks’ gestation Hadlock’s curve was found to underestimate EGA by 3 days, and at 14 weeks Hadlock’s curves were found to overestimate EGA by 2 days. There was no difference between the curves at 9 weeks. These differences probably amount to negligible clinical significance with the exception of clinical decisions that are made at the cusp of fetal viability between 23 and 24 weeks’ gestation.

Regardless of the CRL curve the individual practitioner chooses to use, most authors agree that after 14 weeks multiple biometric parameters should be used to determine EGA when it has not been previously established. Four biometric parameters, the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), have been shown to most accurately predict EGA and EFW. The addition of other parameters does not increase the accuracy of these measurements under normal circumstances. Each measurement will be discussed separately in the upcoming sections.

Biparietal Diameter

The BPD as a predictor of gestational age has been well studied and shown to have good accuracy (±1 wk) as a single measured parameter from 14 to 20 weeks.^38-40 The BPD has also been shown to be as accurate as the CRL between 12 and 14 weeks.^9,25,41-43 Historically, the BPD (Table A6-2) was measured at the widest point of the parietal eminences; however, as variability in head shape is common, the measurement has been standardized. Ideally, the BPD should be measured perpendicular to the midline of the brain through the plane of section that transects the third ventricle and thalami (Figure 6-4). When the measurement is obtained the calvaria should be symmetric and smooth to ensure a true axial plane of imaging, and the cursors should be placed in a “leading edge to leading edge” manner, placing them on the outer edge of the skull in the near field and the inner edge of the skull in the far field (Figure 6-5). In clinical practice, any plane of section transecting the third ventricle and thalami with a symmetric head is acceptable for obtaining the BPD. This is pertinent in numerous situations, not limited to the third trimester when the fetal lie is cephalic and the head is engaged and makes “ideal” measurements difficult, when the fetal head remains in the occiput-anterior or posterior position, or with multiple gestations when one fetus is blocking ultrasound interrogation of another.

The BPD loses its accuracy for gestational age dating when there is a variation in the skull shape as occurs with breech presentation, oligohydramnios, and multiple gestations.^9,38,44-47 In these instances, the head can appear dolichocephalic, or elongated and flattened in the parietal dimension, and this will decrease the BPD measurement (Figure 6-6). To compensate for this in the past, the cephalic index (CI) was calculated. The CI is the ratio of the BPD divided by the occipitofrontal diameter (OFD), and its normal range is 0.75 to 0.85. Historically, before the use of multiple parameters for the determination of EGA, when the CI was close to or beyond either end of the confidence limits, the BPD was not used to assess gestational age. In these cases the HC, which does not change its measurement with changes in the shape of the head, was preferred for establishing EGA. In modern day practice weighted formulas and the inclusion of multiple parameters compensate for most variations, except the most variations, extreme changes in the shape of the fetal head.

As pregnancy progresses the BPD increasingly loses accuracy for the prediction of EGA, with a variability ranging from ±1.4 weeks (2SD) at 14 to 20 weeks to ±4.1 weeks (2SD) at 32 to 42 weeks.^48,49

Head Circumference

As a single measurement the HC which unlike the BPD is independent of head shape, predicts EGA better than the other traditionally used biometric parameters (BPD, AC, FL).^48,50-54 Multiple authors have reported that the HC is the best single predictor of EGA.^48,50,52,54 The HC is optimally imaged through the same transthalamic plane used to image the BPD, but additionally the cavum septi pellucida should be seen in the anterior field, and the tentorial hiatus should be seen posteriorly. Ideally, the angle of insonation should be perpendicular to the falx cerebri, and the intact fetal calvarium should demonstrate an elliptical shape (see Figure 6-4).

The HC is typically measured with electronic calipers (Figure 6-7), but for historical interest the formula for calculating it is shown as follows:

As with BPD, the variation in determining the EGA by using the HC increases as gestation continues. At 14 to 20 weeks the variation is ±1.2 weeks (2SD) and increases to ±3.8 weeks (2SD) by 32 to 42 weeks’ gestation.⁴⁸ A nomogram that allows calculation of gestational age from the head perimeter is provided in Table A6-3.

Abdominal Circumference

Of all the biometric parameters, the AC is the most challenging to image accurately. The AC demonstrates the largest reported variability,^48,52,55,56 and this increased variability is dependent not only on the rate of fetal growth and subcutaneous tissue deposition but also on fetal position and the ability to take accurate measurements.^38,48,50,57 The appropriate plane of measurement is an axial plane maximizing the transverse diameter of the liver (Figure 6-8). The abdomen should be round, there should only be a single rib seen on each side of the abdomen to ensure that the plane is truly axial, and the confluence of the right and left portal veins, known as the “hockey stick,” should be visible. The stomach, if present, should be seen on the left side of the abdomen in this view. An ellipse is then placed around the soft tissue margins of the abdomen to measure the AC (Figure 6-9).

Again for historical interest, the AC can also be calculated by measuring the maximum anteroposterior and transverse diameters of the soft tissue surrounding the abdominal cavity and using the following formula:

Because of the multiple factors affecting the actual size of the AC and the inherent difficulties in accurately measuring it, there is increased variability of the AC for predicting EGA as compared to the other biometric measurements; however values can be used to diagnose growth issues (Table A6-4). Additionally, as with the other biometric parameters the AC has increasing variability as the gestation progresses with a variation of ±2.1 weeks (2SD) at 14 to 20 weeks to ±4.5 weeks (2SD) at 32 to 42 weeks.⁴⁸

Femur Length

Nomograms for all fetal long bones exist, but secondary to the ease in obtaining a satisfactory image the FL has become the standard long bone included in biometric estimation of EGA (see Table A6-4). Although the literature is mixed on the accuracy of the FL to predict EGA, there is good evidence to demonstrate that it is at least as accurate as the other biometric parameters for the determination of EGA.^4,47,58-60 The measurement is obtained by imaging the femur with the angle of insonation perpendicular to the long axis. Only the ossified areas of the femur are measured, but to ensure that the entire length of the long bone is measured, the image should incorporate views of both the cartilaginous femoral head and the femoral condyle (see Figure 6-9). The calipers are placed at the cartilage-bone junction on either end (Figure 6-10). Special care must be taken when obtaining the FL not to measure the “distal femoral point,” an imaging artifact, which appears as a bright spike off of the end of the metaphysis (see Figure 6-9). Again, as with all the other measurements, the variability for predicting EGA widens with increasing EGA from 1.4 weeks (2SD) at 14 to 20 weeks to 3.5 weeks (2SD) at 32 to 42 weeks.⁴⁸