Risk of cesarean delivery when second-trimester ultrasound dating disagrees with definite last menstrual period.

Estimates of gestational age based on early second-trimester ultrasound often differ from that based on the last menstrual period (LMP) even when a woman is certain about her LMP. Discrepancies in these gestational age estimates may be associated with an increased risk of cesarean section and low birth weight. We analyzed 7228 singleton, low-risk, white women from The Routine Antenatal Diagnostic Imaging with Ultrasound trial. The women were recruited at less than 14 weeks of gestation and received ultrasound exams between 15 and 22 weeks. Our results indicate that among nulliparous women, the risk of cesarean section increased from 10% when the ultrasound-based gestational age exceeded the LMP-based estimate by 4 days to 60% when the discrepancy increased to 21 days. Moreover, for each additional day the ultrasound-based estimate exceeded the LMP-based estimate, birth weight was higher by 9.6 g. Our findings indicate that a positive discrepancy (i.e., ultrasound-based estimate exceeds LMP-based estimate) in gestational age is associated with an increased risk of cesarean section. A negative discrepancy, by contrast, may reflect early intrauterine growth restriction and an increased risk of low birth weight.