evidence: Mean (SD) gestational age at the ultrasound visit was 76.1 (12.5) days [10.9 (1.8) weeks] by LMP and 73.3 (12.9) days [10.5 (1.8) weeks] by CRL (p < 0.001), with a mean (SD) difference of 2.8 (10.8) days
intervention: Last menstrual period method
comparator: Crown-rump length method
outcome: Gestational age at birth 