Different ways of estimating the duration of pregnancy can be used. The conceptual age is calculated from the first day of the LMP. Gestational age, which is calculated from theoretical time of ovulation, plus 2 weeks, is generally the accepted parameter. This has the advantage over menstrual age of eliminating the problems associated with oligomenorrhea and delayed ovulation.
The weeks are always counted as completed weeks, not as current weeks. A patient whose LMP started on January 1 therefore will be in her fourth week on February 1.
Parameters Proposed for Gestational Age Assessment
In all the tables in this section, gestational age is expressed in weeks and days from the (theoretical) onset of the normal menstrual period (conception date minus 15 days). Tenths of weeks are not used to avoid difficulties in conversion to days. All measurements are expressed in millimeters instead of centimeters to be consistent with the International System of Units in which secondary units are related to the primary unit by 10 raised to the power of 3 (thousand, million, billion, thousandth, micro, pico, and so forth) and in which the use of centimeters is not recommended.1
The parameters proposed to establish gestational age are detailed below.
Prior to an embryo or yolk sac being seen, an echolucency surrounded by an echogenic ring can be seen on sonography, typically endovaginal probe. Three measurements in orthogonal planes are utilized and a mean diameter is calculated. This mean sac diameter (MSD) is then compared to gestational age tables. The MSD is fairly precise up to 14 mm, but once the embryo can be measured (5 mm), a crown-rump length (CRL) is more accurate. A gestational sac must be differentiated from fluid within the endometrial cavity, the so-called "pseudogestational sac" that may be present with ectopic pregnancies. Table 5-4 compares the characteristics of a true gestational sac versus the pseudosac.
Table 5-4CRITERIA OF TRUE GESTATIONAL SAC VERSUS PSEUDOSAC ||Download (.pdf) Table 5-4 CRITERIA OF TRUE GESTATIONAL SAC VERSUS PSEUDOSAC
|Parameter ||True Gestational Sac ||Pseudosac |
|Shape ||Oval, smooth borders ||Irregular in shape |
|Position ||Eccentric from endometrial stripe ||Within endometrial cavity |
|Sac ||Double decidual sign ||Low-level echo around sac |
The CRL (Table A5-1) is the longest demonstrable length of the embryo or fetus, excluding the limbs and yolk sac.2 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 pathological disorders. Even if differences do occur during that time, they are too small to be detected by ultrasound. Although originally described with static scanners, CRL measurement is much faster and just as reliable when obtained in real time. In a scan that demonstrates a longitudinal section of the fetus, which ideally should be the midline including the fetal spine, the calipers should be placed at the outer edge of the cephalic pole and the outer edge of the fetal rump (Figure 5-3). Because the embryo can display flexion or extension, at least 3 measurements should be obtained and averaged. The limbs and yolk sac should not be included. It is predictive of menstrual age with an error of 3 days (90% confidence limits) from 7 to 10 weeks.
Crown-rump length excluding limbs and yolk sac is measured from the top of the head to the outer edge of the fetal rump.
As growth progresses, however, the curvature of the fetus changes and the linear measurements that can be obtained with the calipers are less accurate. The error increases to 5 days between 10 and 14 weeks of gestation. After 14 weeks gestation, multiple biometric parameters should be used.
Historically, BPD (Table A5-2) was the first parameter used to assess gestational age. Its accuracy is greatest between 12 and 28 weeks. The consensus that has been reached is to measure the BPD at the level of the thalami (Figure 5-4). Measurement rostral to the thalami (below the cerebral peduncles) may result in the underestimation of the BPD and, consequently, of the gestational age. Usually, the BPD is the widest transverse diameter of the fetal skull; however, the transthalamic landmark is more accurate. Biparietal diameter charts are based on measurement from the outer table of the proximal skull to the inner table of the distal skull,3,4 corresponding to the "leading edge to leading edge." Although the BPD may change with the gestation age (and move slightly rostrally), using the same landmark throughout the pregnancy is preferred to avoid unnecessary confusion. It is ill advised to change the plane in late pregnancy because the reference charts were established with the fixed plane described above.
BPD in transthalamic plane is measured from the outer edge of the skull (proximal field) to the inner edge of the skull (far field).
The fetal head can occasionally be flattened and elongated (dolichocephaly) and the BPD thereby artificially decreased (Figure 5-5). To check for this, the cephalic index (CI) should be obtained. The CI is the ratio of the BPD divided by occipitofrontal diameter (OFD), and its normal range is 0.75 to 0.85. When the CI is close to or beyond either end of the confidence limits, the BPD should not be used to assess gestational age. In general, the head circumference (HC) is more accurate than the BPD when the fetal head shape is abnormal.
Fetal head is doilichocephalic (head elongated) in a fetus that has premature rupture of membranes.
Doubilet and Greenes correct the BPD when it is deformed.5 Their procedure is different from the CI. What they have attempted to do is to provide the normal measurement of BPD if the head had not been deformed. They use one of two approaches: either calculate the area of the head and derive the BPD from it or use a circumference correcting the BPD. Although they recommended using the area corrected by BPD, we have been uneasy with that approach because the cross-sectional area of a volume that is compressed is not constant. For example, if you compress a coffee cup between your fingers, the distance between your fingers will decrease, and the cross-sectional area of the cup remains the same. The same is true of the fetal head. If you press the head side to side, the BPD will decrease, the OFD will increase, and the head area will decrease, but the head perimeter will not change. For these reasons we do not recommend using the error-corrected BPD. If either of these methods is used, the perimeter-corrected BPD would probably be of more value. However, in practice, that would increase the number of calculations needed and would not offer significant advantage over the head perimeter method described below.
Wolfson et al showed that, in fetuses that have premature rupture of the membranes, the BPD is not very reliable in assessing gestational age.6 This is no surprise in view of the discussion on dolichocephaly. In fetuses that present for the first time with premature rupture of the membrane, measurement of the femur and humerus should be obtained. An average of these 2 measurements usually provides the best available assessment.
O'Keeffe et al conducted a study similar to that of Wolfson et al. O'Keeffe et al arrived at the same conclusion concerning the use of the BPD and suggested the use of the head perimeter and femur measurements.7 However, O'Keeffe et al also suggested the use of the abdomen perimeter. In view of the fact that premature infants are often delayed in growth, the abdomen perimeter should not be used because the abdomen perimeter could be less than expected, resulting in some underestimation of fetal age.
Because shape, growth disturbances, and individual variation affect the size of the head to an increasing degree after 28 weeks of gestation, the BPD should be used with some caution after this point.8
In the early 1980s, a technique called growth-adjusted sonographic age was proposed to correct the gestational age in serially studied fetuses that were suspected of growth disturbances. Although the technique was an excellent idea from a conceptual viewpoint, it was often impractical to use. It has been shown that the technique did not offer any advantage over a single BPD measurement in a population of well-controlled patients.9
Head Perimeter or Head Circumference
The HC is optimally imaged through the thalami and cavum septi pellucida, with the falx cerebri perpendicular to the insonation beam (falx horizontal on image). When the falx is parallel to the scanning plane, turning the patient to the side or waiting a short time may be helpful. A transthalamic plane showing intact fetal calvarium in an elliptical shape, absence of fetal cerebellum, or fetal orbits is optimal. The head perimeter is influenced by growth disorders but to a lesser extent than the BPD. Ott compared the mean error of the head perimeter to the mean error of BPD and found it to be significantly smaller.10 It is not influenced by dolichocephaly or brachycephaly. The head perimeter is measured in the same transthalamic plane as the BPD, and in general should be measured in the same image used for the BPD. One should make certain that the longest (anteroposterior) length is obtained, which implies that the cavum pellucidum or the roof of posterior fossa is included in the scan. Sometimes the indistinct calvarium of the far-field skull may cause an inadvertent overmeasuring; careful attention to the calvarium thickness such as in the near field allows for correct placement of the calipers around the entire skull. In a laboring patient and in some circumstances of fetal head engagement, a BPD or an HC measure is not possible, and a suboptimal measure should not be used. The head perimeter is either measured with electronic calipers (see Figure 5-6) or computed by using the formula:
A nomogram that allows calculation of gestational age from the head perimeter is provided in Table A5-3.
Head perimeteris a circumferential measure of the fetal head at the transthalamic level.
Femur and Humerus Lengths
Femur length (Table A5-4) was originally measured to diagnose limb dwarfism. It was subsequently observed that femur length was an excellent parameter to determine fetal age.3,11,12,13, and 14 The femur can be measured from 10 weeks onward. The femur is measured from the origin to the distal end of the shaft (Figure 5-7). The femoral head and distal epiphysis are not included in the measurement. The optimal technique is to measure the femur closest to the near field, with the FL perpendicular to the insonation beam (horizon rather than vertical), and equal echogenicity throughout its length. A common imaging error of the FL is to demonstrate an incomplete bony diaphysis. Noting distinct shadowing on both ends is sometimes helpful in assuring a correct measurement (see Figure 5-7). The femoral head and distal epiphysis are not included in the measurement. The humerus is measured in the same way (Figure 5-8). The humerus (Table A5-4) is also commonly measured.
Femur length. The femur length is obtained from the most proximal portion of the shaft to the distal end. Neither the femoral head nor the distal epiphysis is included. The proximal femur should be imaged rather than the femur in the far field.
Humeral length. The humerus is measured in the same manner as the femur length.
Crown-rump length, BPD, and femur and humerus lengths are by no means the only parameters useful in estimating gestational age. Among the others that have been proposed are other fetal long bones,14 binocular distance,15 head perimeter,16,17 the abdominal perimeter,16 clavicle, and the size and shape of the fetal ears.18 The abdominal perimeter has rapidly been abandoned because it is too sensitive to variations of fetal growth.
Among the other long bones, measurement of the humerus is generally the easiest to obtain and the most reproducible. The tibia comes next; the radius and ulna should be used only when confusing results are obtained from other methods. In practice, the bone-derived gestational ages are averaged and compared with the BPD-derived gestational age. If the difference is greater than 11 days, using the bone-derived gestational age is preferred. Table A5-5 (humeral and ulnar length and tibia and clavicular length may be used to determine the gestational age from the long bones of the fetus (Table A5-6).
The binocular distance is a parameter that has occasionally proven useful. To obtain the right plane, one should start from the conventional section of the BPD and move the transducer caudally until the orbits are visualized. In the correct plane, both eyes should have the same diameter, and the image should be symmetrical (Figure 5-9). The largest diameter of the eye should be used; the interocular distance should be the smallest. Table A5-7 can be used to derive the gestational age from the binocular distance.
Measurement of binocular distance. Both eyes should have the same diameter, and the measurement is obtained from the interface between the globe and the orbit on either side. The interocular diameter (IOD) is the distance between the medial border of one orbit and the medial border of the opposite side (A); the binocular diameter (BOD) is the distance between the lateral border of one orbit and the lateral border of the opposite side (B).
The cerebellar measurement was proposed by Reece et al,18 who found that the measurement of the cerebellum correlated well with the gestational age and was not much affected by growth retardation (Figure 5-10). Further, if the measurement is expressed in millimeters, the value is very similar to the gestational age. For instance, if a cerebellum is measured to be 22 mm, the fetus is about 22 weeks in gestational age. This similarity in value makes it easy to use the cerebellum as a rapid check of the gestational age.
Cerebellar diameter is a transverse measure of the cerebellar hemispheres.
The clavicle is also proposed as a measurement to arrive at gestational age. The clavicle has an intramembranous ossification and not an endochondral ossification. This differentiates it from other long bones of the body. The clavicles are affected to a very different degree by diseases such as achondroplasia, thanatophoric dysplasia, and so forth. The clavicle is occasionally useful in such circumstances. Another interesting point is that the length of a clavicle expressed in millimeters is very close to the gestational age expressed in weeks.19
Many investigators have also attempted to determine the gestational age based on identification and measurement of ossification centers such as the distal femoral, proximal tibial, and proximal humeral. This method has been associated with a few problems. The first is that the observation of an ossification center by itself is not sufficiently precise. When only the presence of the ossification center is assessed, the fetus can be any age after the age at which the ossification center appears, information that is of little use in clinical practice.
The measurement of the ossification center is also a problem. What appears as the ossification center on ultrasound is only the interface on the proximal side of the center of the ossification center and not the whole ossification center. The thickness and length are not visible unless the ossification center lies exactly perpendicular to the ultrasound beam. This position is difficult to obtain in clinical practice. The ossification centers are small, and a small error in measurement on any of them is equivalent to a large absolute variation in the assessment of gestational age. Therefore, the use of the ossification center is of little value in clinical practice; even in late gestation, the measurement of the femur and humerus is preferred.20
Selection of an Appropriate Table
In view of the large number of different parameters, it is important to know which one to measure and when. Different parameters have different reliability and ease of measurement at different gestational ages. In the list that follows, the parameters are given in decreasing order of preference. This order was established from their reliability, confidence limits, and ease of measurement. It should not be regarded as definitive and can be adapted to specific circumstances.
From 7 to 10 weeks: CRL
From 10 to 14 weeks: CRL, BPD, FL humerus length
From 15 to 28 weeks: BPD, FL, humerus length, head perimeter, binocular distance, other long bone lengths
After 28 weeks (more accurate for dating): femur length, humerus length, binocular distance, BPD (check that BPD is correct by evaluating the CI), other long bone lengths, head perimeter
When Different Parameters Have Discrepancies in Estimates
The general rule is that the earlier the estimation of the age, the more accurate it is. In other words, the first ultrasound typically establishes the dates. The exception is that with an extremely early gestational age, there can be errors of the measuring technique (ie, calipers are bigger than the distance measured). If estimate of the fetal age has been made at 15 weeks and a follow-up scan at 27 weeks yields a different estimate, do not change the original estimate—it is more accurate. Rather, the follow-up sonogram should indicate fetal growth. Ages are more or less equivalent when they are within 11 days of each other (this is an arbitrary limit). Before 20 weeks, parameters should be remeasured when the difference exceeds 1 week. Clinical information should also be used in assigning gestational age such as the certainty of the LMP, maternal or fetal conditions that affect the biometric parameters (such as gestational diabetes increasing the fetal abdominal circumference [AC]), and the gestational age of the baby.
One important time period to consider is the third trimester, where the rule of thumb is to be very cautious in changing the gestational age, because there is greater fetal biological variation. A growth-restricted fetus may be erroneously assigned a lesser gestational age. In general, with a discrepancy in LMP-ultrasound, the dates should not be definitively assigned on the basis of one ultrasound, but rather a follow-up sonogram can be helpful: a follow-up showing normal fetal growth would be consistent with wrong dates, whereas a follow-up showing little growth would be consistent with fetal growth restriction.
Careful methodology with multiple measures should be used. Although it is reassuring when two similar parameters agree (eg, 2 different bones), the sonographer and sonologist should refrain from "forcing" an image to meet a predicted expectation. When parameters do agree, an average gestational age can calculated. When a few parameters provide estimates that are in the same range with only one discordant, reassess the outlier for methodological errors. If it is still abnormal, do not include it in the final estimate, and consider the reason for its disparity.