How Is the Due Date Calculated?

Although there is considerable variability among women, labor typically occurs between weeks 38 and 42 of pregnancy.

In normal pregnancies, the most common method for estimating the due date is using pregnancy calculators, which rely on the date of the last menstrual period (LMP). To estimate the expected delivery date, count 40 weeks from the first day of your last menstrual period. Once you have this date, you can establish a high-probability window for delivery, ranging from two weeks before to two weeks after the estimated date (corresponding to the period between 38 and 42 weeks of gestation).

For women with irregular menstrual cycles, in addition to using the LMP, the crown-rump length (CRL) measurement taken during the first trimester is commonly used to estimate more accurately the gestational age and, therefore, the due date.

For pregnancies achieved through embryo transfer, a "theoretical" LMP is calculated by subtracting 19 days from the embryo transfer date for a blastocyst, or 17 days for a day-3 embryo.

It is important to note that the due date is only an estimate. Only about 5% of women deliver on their exact due date.