First- and Second-trimester Ultrasound Assessment of Gestational Age

Objective: The purpose of this study was to evaluate and compare the accuracy of first- and second-trimester ultrasound assessment of fetal age.

Study Design: Differences between ultrasound estimated gestational age and true gestational age were calculated between 11 to 14 weeks of gestation and 18 to 22 weeks of gestation for singleton, twin, and triplet pregnancies that were conceived by in vitro fertilization.

Results: One hundred four singleton pregnancies, 81 twin pregnancies, and 33 triplet pregnancies were included. First trimester ultrasound estimates overestimated gestational age by a mean (+/- SEM) of 1.3 +/- 0.2 days (P < .0001) for singleton pregnancies, 1.4 +/- 0.2 days (P < .0001) for twin pregnancies, and 0.8 +/- 0.4 days (P = .027) for triplet pregnancies. Second trimester estimates were underestimated by a mean of -0.1 +/- 0.4 days (P = .87) for singletons, -0.6 +/- 0.3 days (P = .083) for twins, and -0.6 +/- 0.5 days (p = 0.21) for triplets.

Conclusion: Ultrasound assessment of gestational age is very accurate and is marginally better in the first trimester compared with the second.