On completion of this chapter, you should be able to:
Discuss gestational sac growth, take measurements, and assess gestational age
Describe how to perform a crown-rump measurement and evaluate growth
Calculate the biparietal diameter, head circumference, three-dimensional cranium, abdominal circumference, and extremity measurements
Assess fetal parameter measurements, proportions, and fetal growth
Describe when other measurements should be used to provide additional clinical information
Evaluate the fetal growth time-series for intrauterine growth restriction and growth disturbances
Reliably assessing gestational age and the growth of the fetus has long posed a challenge to all who care for pregnant women. Although not without value, clinical parameters lack the necessary consistency for optimal perinatal care. With recent advances in diagnostic sonography, however, fetal age and growth can be assessed with high accuracy.
The difference between menstrual age and fetal age is important to establish in clinical obstetrics. Fetal age begins at the time of conception and is also known as conceptional age. Conceptional age is restricted to pregnancies in which the actual date of conception is known, as found in patients with in vitro fertilization or artificial insemination. If conceptual age is already known, the menstrual age may be found by adding 14 days to the conceptual age.
Obstetricians date pregnancies in menstrual weeks, which are calculated from the first day of the last menstrual period (LMP) or last normal menstrual period (LNMP). This method is called menstrual age or gestational age. The student of sonography must understand that gestational ages are estimated from measurements of fetal parameters and are not the actual age of the parameters. The estimated ages are no more accurate than the measurements taken, and all parameters in a fetus will not result in the same fetal age because fetuses have different proportions: some are long; some are short; some are fat; some are thin.
It is clinically important to know the menstrual age of a patient because this information is used for the following reasons:
In early pregnancy to schedule invasive procedures (chorionic villus sampling and genetic amniocentesis)
To interpret maternal serum alpha-fetoprotein screening
To plan date of delivery
To evaluate fetal growth
Before the use of sonographic determination of fetal growth, menstrual age was calculated by three factors: (1) the menstrual history, (2) physical examination of the fundal height of the uterus, and (3) postnatal physical examination of the neonate. This process was not always reliable if the patient could not recall the date of her last period or if other factors—such as oligomenorrhea, implantation bleeding, use of oral contraceptives, or irregular menstrual cycle—were present.
Because all gestations do not deliver exactly at 40 weeks post-LMP, specific terminology has been defined for the relative gestational age at term ( Box 52-1 ).
Early term: From 37 weeks 0 days to 38 weeks 6 days
Full term: From 39 weeks 0 days to 40 weeks 6 days
Late term: From 41 weeks 0 days to 41 weeks 6 days
Postterm: From 42 weeks 0 days to delivery
Gestational age assessment: First trimester
Gestational sac diameter
Transvaginal sonography enables visualization and evaluation of intrauterine pregnancies earlier than was previously thought possible. The earliest sonographic finding of an intrauterine pregnancy is thickening of the decidua. Sonographically, this appears as an echogenic, thick filling of the fundal region of the endometrial cavity occurring at approximately 3 to 4 weeks after the LNMP ( Box 52-2 , Figure 52-1 ).
A distended urinary bladder affects the gestational sac measurement; it changes its shape from round to ovoid or teardrop.
If the sac is round, measure one GSD diameter inner to inner (all three diameters equal).
If the sac is ovoid, make two measurements inner to inner, one transverse and the other perpendicular to the length. Take the third GSD inner to inner.
The three GSD diameters are averaged for the diameter of record.
At approximately 4 weeks of menstrual age, a small hypoechoic area appears in the fundus or midportion of the uterus, known as the double decidual sac sign. As the sac embeds further into the uterus, it is surrounded by an echogenic rim and is seen within the choriodecidual tissue. This is known as the chorionic or gestational sac.
At 5 weeks after the LMP, the average of the three perpendicular internal diameters of the gestational sac—calculated as the mean of the anteroposterior diameter, the transverse diameter, and the longitudinal diameter—can provide an adequate estimation of menstrual age (see Figure 52-1 ). A gestational sac should be seen within the uterine cavity when the beta–human chorionic gonadotropin (beta-hCG) is above 500 mIU/ml (Second International Standard). This becomes especially important when evaluating a pregnancy for ectopic implantation.
The sac grows rapidly in the first 10 weeks, with an average increase of 1 mm per day. According to one report, a gestational sac growing less than 0.7 mm per day is associated with impending early pregnancy loss. Even the most experienced sonographer may incorporate a measuring error; therefore the beta-hCG test in conjunction with a sonographic evaluation is suggested in a sequential time frame.
When the gestational sac exceeds 8 mm in mean internal diameter, a yolk sac should be seen. The yolk sac is identified as a small, spherical structure with an anechoic center within the gestational sac. It provides early transfer of nutrients from the trophoblast to the embryo. It also aids in the early formation of the primitive gut and vitelline arteries and veins and in the production of the primordial germ cells. Yolk sac size has not been correlated with gestational age determination. Normal yolk sac size should be less than 5.5 mm. Yolk sacs greater than 5.6 mm have been associated with poor pregnancy outcome, as have solid, echogenic yolk sacs. Box 52-3 lists sonographic landmarks for early pregnancy.
500 mIU/ml beta-hCG = gestational sac seen
>8-mm gestational sac = yolk sac seen
>16-mm gestational sac = embryo seen
<6-mm yolk sac = normal
>8-mm yolk sac = abnormal
>7-mm fetal pole = positive cardiac activity
When the mean gestational sac diameter (GSD) exceeds 16 mm, an embryo with definite cardiac activity should be well visualized with transvaginal scanning. This usually occurs by the sixth menstrual week ( Figures 52-2 ) but may be as early as the fifth LMP week with transvaginal sonography. For the transabdominal scanning approach, the maternal urinary bladder must be filled to create an acoustic window. With this technique, the sac shape can vary secondary to bladder compression, maternal bowel gas, or myomas and should not be misinterpreted as abnormal.
Assessing gestational age using a single gestational sac diameter or even up to three averaged diameters yields an accuracy of only ±2 to 3 weeks in 90% of cases. Accordingly, gestational sac diameter has not been widely used as a determinant of gestational age after more accurate embryonic parameters can be measured (see Chapter 49 ).
With transvaginal sonography, embryonic echoes can be identified as early as 38 to 39 days of menstrual age ( Box 52-4 , Figure 52-3 ; also see Figure 52-2 ). The crown-rump length (CRL) is usually 1 to 2 mm at this stage. The embryo is usually located adjacent to the yolk sac. A first-trimester CRL is the most accurate sonographic technique for establishing gestational age in the first trimester. The reason for this high accuracy is the excellent correlation between fetal length and age in early pregnancy because pathologic disorders minimally affect the growth of the embryo during this time.
By transvaginal sonography, the CRL can be measured from 6 to 12 gestational weeks.
Measurements should be made along the long axis of the embryo from the top of the head (crown) to the bottom of the trunk (rump).
This is the most accurate fetal age measurement.
The embryo can be measured easily with real-time dynamic imaging. For transabdominal imaging, the mother’s bladder should be full to create an acoustic window. The measurement should be taken from the top of the fetal head to the outer fetal rump, excluding the fetal limbs or yolk sac. The accuracy is ±5 days with a 95% confidence level ( Figure 52-4 ). The average of three separate measurements of the CRL may be obtained to determine gestational age.
Cardiac activity should be seen when the CRL exceeds 7 mm, but it may be seen by transvaginal sonography once the CRL reaches 2 mm. It is generally accepted that it is a good idea to follow patients with small CRL and no fetal heartbeat over a few days. In general the CRL should increase at a rate of 8 mm per day. Occasionally an embryo is seen with no visible cardiac activity and a small CRL for menstrual age. It is advisable to wait a week and rescan to see if the patient spontaneously aborts the products of conception or if she needs medical intervention, such as a dilation and curettage procedure. Infrequently an appropriate fetal CRL and positive cardiac activity are seen after the week’s wait. Why this happens is not known, but experienced sonographers have observed it.
Absence of an embryo by 7 to 8 weeks of gestation is consistent with an embryonic demise or an anembryonic pregnancy. If a nomogram is not readily available to identify gestational age, a convenient formula is gestational age in weeks = CRL in cm + 6. After the 12th week, a CRL is no longer considered accurate because of flexion and extension of the active fetus; therefore other biometric parameters should be used.
Embryonic heart rate
The embryonic heart rate (EHR) accelerates linearly during the first month of beating between the fifth and ninth gestational weeks ( Figure 52-5 ). This linear acceleration correlates well with the embryonic age before the CRL reaches 2.5 cm or before approximately 9.2 LMP weeks. The mean rate of acceleration is 3.3 beats per minute per day, 10 beats per minute every 3 days, or approximately 100 beats per minute between the start of beating until the early ninth week ( Box 52-5 ). The embryonic age in days can be estimated with the following formula:
L M P age in days = EHR × 0 .3 + 6 days
By transvaginal sonography, the EHR can be measured by M-mode to estimate age from the early fifth to early ninth gestational weeks when the CRL is less than 25 mm.
The most accurate EHR measurements can be obtained by enlarging the 2D image and using a fast sweep speed for the M-mode tracing. Measure from repeating distinct points on the M-mode.
The EHR age will be accurate to within ±6 days. An EHR age that trails the CRL age by more than 6 days may be associated with impending first-trimester failure and warrants follow-up.
The normal EHR accelerates linearly at 3.3 beats per minute per day, which is approximately 10 beats every 3 days, or 100 beats in the first month of beating.
The result of this estimation will be within ±6 days in 95% of normal pregnancies. If the age estimated by the CRL leads the age by EHR by more than 1 week, it may be prognostic for first-trimester failure and warrants follow-up. Because the heart rate is accelerating so rapidly during the embryonic period, accurate M-mode measurements are desirable. The greatest accuracy can be achieved by magnifying the embryo as much as possible and using a fast M-mode tracing to stretch out the heartbeats for more precise cursor placement. The cursors should be carefully placed on identifiable, repeating locations of the M-mode tracing.
Gestational age assessment: Second and third trimesters
In the second trimester, the gestational age parameters extend to the biparietal diameter, head circumference, abdominal circumference, femur length, and other parameters that may be used. It is critical for the sonographer to know precisely which landmarks are necessary to determine these measurements. Proper gain settings, instrumentation, and fetal lie all influence the accuracy of measurements used to estimate the gestational age.
In the second trimester, the biparietal diameter (BPD) was the first and is the most widely accepted means of measuring the fetal head and estimating fetal age. As the pregnancy enters the third trimester, an accurate measurement of fetal age becomes more difficult to obtain because the fetus begins to drop into the pelvic outlet cavity. The reproducibility of the BPD is ±1 mm (±2 standard deviations). When dating a pregnancy between 17 and 26 weeks of gestation, the predictive value is ±11 days in 95% of the population. After 26 weeks, the correlation of BPD with gestational age decreases because of cranial molding and the increased biologic variability. The predictive value decreases to ±3 weeks in the third trimester. The growth of the fetal skull slows from 3 mm per week in the second trimester to 1.8 mm per week in the third trimester.
When measuring the BPD, it is important to determine the landmarks accurately ( Box 52-6 ; Figures 52-6 and 52-7 ). The fetal head should be imaged in a transverse axial section, ideally with the fetus in a direct occiput transverse position. The BPD should be measured perpendicular to the fetal skull at the level of the thalamus and the cavum septi pellucidi. Intracranial landmarks should include the falx cerebri anteriorly and posteriorly, the cavum septi pellucidi anteriorly in the midline, and the choroid plexus in the atrium of each lateral ventricle ( Figure 52-8 ). With real-time sonography, one can identify the middle cerebral artery pulsating in the insula.
Obtain biparietal leading edge–to–leading edge diameter (BPD) of the fetal head at the transverse level of the midbrain: falx, cavum septi pellucidi, and thalamic nuclei.
Make sure the head is symmetric and oval.
Measure from the outer to the inner margins of the skull.
In the third trimester, the BPD is not as accurate in predicting fetal age; may approach ±3 to 3.5 weeks.
The head shape should be ovoid, not round ( brachycephaly ), because this can lead to overestimation of gestational age, just as a flattened or compressed head ( dolichocephaly ) can lead to underestimation of gestational age estimated from the BPD measurement. The calipers should be placed from the leading edge of the parietal bone to the leading edge of the opposite parietal bone (known as “outer edge to inner edge”). The parietal bones should measure less than 3 mm each. On the outer edge of the fetal head, the soft tissue should not be included; measuring should begin from the skull bone, excluding the scalp. Gain settings should not be set too high because this can produce a false thickening and incorrect measurement. A reference curve should be applicable to the local population. BPD should not be used to date a pregnancy in cases of severe ventriculomegaly, which may alter the head size and produce macrocephaly, or when microcephaly, skull-altering head lesions, or molding is present. In these cases, other biometric parameters should be used.
If the fetus is too large for an accurate CRL but too early for a BPD with the proper landmarks identified, an approximation of the BPD can be obtained by incorporating the following landmarks: a smooth, symmetric head; visible choroid plexuses; and a well-defined midline echo that is an equal distance from both parietal bones ( Figure 52-9 ).
One technique to adjust for the biologic variability of fetal head growth uses the fetus as its own control. Two separate BPD measurements are obtained, the first between 20 and 26 weeks and the second between 31 and 33 weeks. The growth interval was compared with average growth. This technique has been termed growth-adjusted sonar age (GASA). The fetus is then categorized into a small, average, or large growth percentile. The developers of this method claim the use of GASA reduces the range in gestational age from ±11 days to ±3 days in 90% of fetuses and to ±5 days in approximately 97% of fetuses. Although GASA compensates for the biologic variability in the individual fetus, it does not take into consideration other factors, such as dolichocephaly, brachycephaly, and oxycephaly caused by molding, variations in head shape, or the standard error of measurement.
Prenatal compression of the fetal skull is common. It occurs more often in fetal malpresentation, such as breech, or in conditions of intrauterine crowding, such as multiple pregnancies. The fetal skull can also be compressed in vertex presentations without any obvious reason or as a result of an associated uterine abnormality, such as leiomyoma. The transverse head circumference (HC) is less affected than the BPD by head compression, so the HC is a valuable tool in assessing gestational age ( Figure 52-10 ; also see Box 52-7 ).
Use the transverse plane at the level of the BPD to calculate HC.
Place area calipers along the outer margin of the skull to obtain circumference.
Accurate to ±2 to 3 weeks.
The HC measurement is taken in the transverse plane at the level of the BPD and can be calculated from the same frozen image. Most modern sonographic equipment has built-in electronic calipers that open to the outline of the fetal head. If this feature is not available, the measurement can be obtained with a light pen, map reader, or electronic planimeter or by manually measuring with the electronic calipers. To measure the widest transverse diameter of the skull manually the BPD level should be measured from the outer skull, not including scalp, to the inner table of the distal skull (see Figure 52-6 ). Note that the BPD is not the diameter of the outer cranial perimeter. The length of the occipital frontal diameter (OFD) is measured as the longest skull diameter. The transverse HC is done by elliptical calipers around the outer perimeter of the cranium measuring from the outer border of the occiput to the outer border of the frontal bone (see Figures 52-6 and 52-10 ). The HC can be calculated manually by the following formula:
HC = BPD + OFD × π 2
HC = (BPD + OFD) × 1 .57
Two frequently noted alterations in head shape are dolichocephaly and brachycephaly. In dolichocephaly, the head is shortened in the transverse diameter (BPD) and elongated in the anteroposterior diameter (OFD) ( Figure 52-11 ). In brachycephaly the head is elongated in the transverse diameter (BPD) and shortened in the anteroposterior diameter (OFD) ( Figure 52-12 ). One can underestimate gestational age from a dolichocephalic head or overestimate with brachycephaly. Because of these variations in fetal head shape, a cephalic index (CI) has been devised to determine the normality of the fetal head shape:
CI = BPD/OFD × 100