What is the difference between gestational and conceptional age -- why the two weeks?

Answer 1

The simple answer is that gestational age is calculated from the first day of the pregnant female's last menstrual period, while the conceptional age is calculated from the time of ovulation. In a normal menstrual cycle, the first date of menstruation reliably occurs about 14 days after ovulation, accounting for the two weeks' difference in gestational age and conceptional age.

And now for some background information. Both gestational age and conceptional age are terms used to describe the age of a fetus during a pregnancy, particularly the age at delivery. But as you've observed, they are off by two weeks, such that gestational age is about two weeks greater than conceptional age.

Gestational age is how much time has passed since the first day of the last menstrual period (LMP). Conceptional age is how much time has passed since conception (fertilization). Conception is presumed to have occurred on the day of ovulation. The difference in time between the LMP and ovulation is two weeks, which accounts for the two week difference in gestational age and conceptional age. While individual female's menstrual cycles vary by length, timing, etc., a female reliably begins her menstrual period 14 days after ovulation.