Obstetric Measurements and Gestational Age

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 normal LMP ( Box 52.2 , Fig. 52.1 ).

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 ( Fig. 52.2 ).

At 5 weeks after the LMP, the average of the three perpendicular internal diameters of the GSD—calculated as the mean of the anteroposterior diameter, the transverse diameter, and the longitudinal diameter—can provide an adequate estimation of menstrual age ( Fig. 52.3 ). A gestational sac should be seen within the uterine cavity when the β-human chorionic gonadotropin (β-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/day (see Table 52.1 ). According to one report, a gestational sac growing less than 0.7 mm/day is associated with impending early pregnancy loss. Even the most experienced sonographer may incorporate a measuring error; therefore, the β-hCG test in conjunction with a sonographic evaluation is suggested in a sequential time frame.

A yolk sac should be seen when the gestational sac exceeds 8 mm in mean internal diameter ( Fig. 52.4 ). The yolk sac is 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 producing the primordial germ cells. Yolk sac size has not been correlated with gestational age determination. Normal yolk sac size should be less than 6 mm. Yolk sacs greater than 8 mm have been associated with poor pregnancy outcomes and solid, echogenic yolk sacs. Box 52.3 lists sonographic landmarks for early pregnancy.

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 6th menstrual week ( Fig. 52.5 ) but may be as early as the fifth LMP week with transvaginal sonography. The maternal urinary bladder must be filled to create an acoustic window for the transabdominal scanning approach. 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 Chapters 49 and 53 ).

Crown-Rump Length

With transvaginal sonography, embryonic echoes can be identified as early as 38 to 39 days of menstrual age ( Box 52.4 ). The crown-rump length (CRL) is usually 1 to 2 mm at this stage. The embryo is usually located adjacent to the yolk sac. The CRL is the most accurate sonographic technique for establishing gestational age in the first trimester ( Fig. 52.6 ). 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.

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 ( Table 52.2 and Fig. 52.7 ). The accuracy is ±5 days with a 95% confidence level. 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 good 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/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 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 (weeks) = CRL (cm)+6. After the twelfth week, 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 5th and 9th gestational weeks ( Fig. 52.8 ). 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/min per day, 10 beats/min every 3 days, or approximately 100 beats/min 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:

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 ( Fig. 52.9 ).

Fetal Measurements

In the second trimester, the gestational age parameters extend to the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), femur length (FL), 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.