Estimate gestational age from ultrasound sac measurements.
GA (days) = MSD (mm) + 30. MSD is the average of three orthogonal sac dimensions. A yolk sac should be visible when MSD ≥ 20 mm, and an embryo with cardiac activity when MSD ≥ 25 mm. If MSD ≥ 25 mm with no embryo, the diagnosis of early pregnancy failure can be made.