A 26-year-old woman, gravida 1 para 0, at 32 weeks gestation comes to the office to establish prenatal care after recently moving. She reports no complications in this pregnancy. Blood pressure is 110/60 mm Hg and pulse is 90/min. On examination, fundal height is 26 cm and fetal heart rate is 140/min. Ultrasound calculates the gestational age as 28 weeks based on the fetal biparietal diameter, abdominal circumference, and femur length. On review of records, the patient had very irregular menses prior to pregnancy, and at her first prenatal appointment, she thought she was 6 weeks gestation based on her last menses. However, ultrasound at that visit measured the fetus at 8 weeks gestation, which adjusted the estimated due date. Given the multiple discrepancies in gestational age, which of the following is the best next step in management of this patient?
Ultrasound assessment of gestational age | ||
Ultrasound parameter | Gestational age (weeks) | Accuracy (days) |
Gestational sac diameter | 4.5-6 | ±5-7 |
Crown-rump length | 7-10 | ±3 |
11-14 | ±5 | |
Biparietal diameter, head circumference, femur length | 14-20 | ±7 |
21-30 | ±14 | |
>30 | ±21-28 | |
Pregnancy dating and estimated gestational age (EGA) are used for fetal monitoring and testing and help determine timing of fetal interventions (eg, corticosteroids) and delivery.
Pregnancy dating is often confirmed with a first-trimester ultrasound showing the fetal crown-rump length measurement, which is accurate within 3 days of the gestational age. First-trimester ultrasound is particularly important for estimating gestational age in patients with irregular menses, as in this patient, and can reestablish the estimated due date (Choice B). Although ultrasounds are used throughout pregnancy, they can become less accurate as the pregnancy advances (±3 weeks in the third trimester) due to biologic variation and environmental in utero factors (eg, gestational diabetes, uteroplacental insufficiency). Therefore, once established, EGA is not changed based on later ultrasound estimates (Choices C and E).
This patient who is at 32 weeks gestation based on a first-trimester ultrasound is now measuring at 28 weeks gestation, which is greater than the expected ultrasound variance in this trimester (±3 weeks). Because this pregnancy was dated by a first-trimester ultrasound, the discrepancy between current gestational age and previous ultrasound findings represents abnormal fetal growth, not an inaccurate EGA. Therefore, this patient requires additional work-up for fetal growth restriction (eg, chromosomal studies, infectious etiology [TORCH titers]) and management with serial growth ultrasounds, antenatal testing (eg, biophysical profile), and umbilical artery dopplers.
(Choice A) After 20 weeks gestation, a fundal height measurement is used to screen for abnormal fetal growth (eg, fetal growth restriction, macrosomia); however, it is not used for gestational age estimation because it can be confounded by a variety of factors (eg, fibroids, obesity). When the measurement differs from gestational age by ≥3 cm, an ultrasound is required for additional evaluation.
Educational objective:
Ultrasound dating with fetal crown-rump measurement in the first trimester is the most accurate way to determine estimated gestational age (EGA). EGA should not be changed based on measurement discrepancies on a second- or third-trimester ultrasound.