2026 Abdomen, Vascular, OB/GYN Complete Practice Test – WCUI/Smith Chason Exit Exam Preparation

Dating a pregnancy after the first trimester relies on fetal measurements that track gestational age with consistent growth. The biparietal diameter is the most reliable for dating in this period because the width of the head, measured across the skull at a standardized plane, increases at a steady rate and correlates closely with gestational age from early second trimester onward. This measurement is reproducible when taken at the level of the thalami and cavum septi pellucidi, using the outer-to-inner table of the skull and calipers.

Crown-rump length is limited to dating earlier in pregnancy, since the crown and rump relationship becomes less predictive after the first trimester. Femur length and abdominal circumference are valuable for assessing growth and development, but they introduce more variability for dating purposes; limb length and abdominal size can be influenced by normal individual differences and fetal growth patterns, making them less precise for determining gestational age compared to head measurements in this window.

So, biparietal diameter stands out as the best dating metric after the first trimester because it provides a stable, age-related growth index that remains relatively consistent across fetuses.