Two Ways to Figure the Due Date
Most women don't know the exact date their baby was conceived, but they usually know the day their last menstrual period began. The doctor subtracts 2 weeks from the date of the last period as an estimate of when conception occurred. Your estimated due date is 38 weeks after the date of conception (40 weeks after the first day of your last period).
There is a second way to determine your due date. Add 7 days to the date of the beginning of your last menstrual period, then subtract 3 months. This gives you the approximate date of delivery. For example, if your last period began on January 20, your estimated due date is October 27.
Gestational Age and Fertilization Age
also called menstrual age, dates a pregnancy from the first day of the last menstrual period. It is 2 weeks longer than the fertilization age.
also called ovulatory age, is 2 weeks shorter than gestational age and dates from the actual date of conception. This is the actual age of the fetus.