In cases where the estimated date of delivery is calculated based on menstrual cycle data (see the American College of Obstetricians and Gynecologists' [ACOG] Methods for Estimating Due Date), the ACOG recommends using the ultrasonographic date if it differs from the calculated date by At ≥ 8 6/7 weeks gestation: > 5 days At 9 to 13 6/7 weeks gestation: > 7 days Because ultrasonographic estimates are less accurate later in pregnancy, second and third trimester ultrasonographic results should rarely be used to revise those done during the first trimester.