Gestational age (GA) is the standard measure of pregnancy duration, counted in weeks and days from the first day of the last menstrual period (LMP), even though conception typically occurs about two weeks later. In first trimester ultrasound, gestational age is most accurately estimated by crown-rump length (CRL). In the second trimester, biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL) are used together to calculate estimated gestational age.