Algorithms for combining menstrual and ultrasound estimates of gestational age: consequences for rates of preterm and postterm birth.

We compared rates of preterm and postterm birth according to six algorithms for gestational age (GA) estimates based on last menstrual period (LMP) and early ultrasound (EUS): LMP alone, LMP if the discrepancy between the two estimates was within 14 days and otherwise EUS (14-day rule), a 10-day rule, a seven-day rule, a three-day rule and EUS alone. In a sample of 44,623 births in a Canadian tertiary hospital, the choice of algorithms makes a substantial impact on both preterm and postterm birth rates, even when EUS was used for discrepancies over two weeks.