Most third trimester ultrasounds compare fetal measurements to the weeks of gestational age to which those measurements correspond.
Some women have irregular cycles in which it is much more difficult to calculate both the date of ovulation (and conception) and the probable date of delivery (EDD). In these cases, an ultrasound scan between 8-18 weeks can help calculate the due date, based on the characteristics of the embryo, and thus determine (with a margin of error of four days) the date on which the egg was fertilized.
By Romina Packan (invitra staff).
Last Update: 10/21/2019
