Pregnancy Calculator

Gestational Age vs Fetal Age

Gestational age is the number of weeks since the first day of your last menstrual period (LMP). Fetal age (embryonic age) is typically about two weeks less, counting from conception. Doctors usually use gestational age to track pregnancy.

How the Due Date is Calculated

By default, a due date is estimated as 40 weeks (280 days) from the first day of the LMP (Naegele’s rule: add one year, subtract three months, and add seven days). If you know the conception date, the due date is about 38 weeks (266 days) from conception. Cycle length adjustments can slightly change the result.

Accuracy of the Due Date

Remember, due dates are estimates. Only about 4% of births occur on the exact due date. Roughly 60% of pregnancies deliver within one week of the estimated date, and about 90% within two weeks. Early first-trimester ultrasound is generally the most accurate method to date a pregnancy.

In Vitro Fertilization (IVF)

For IVF pregnancies, the embryo’s age and transfer date are known, so we calculate the due date by adding 266 days to the transfer date for a day-5 embryo (adjusting if the embryo age is 3 or 6). This approach yields a precise due date since the timing of conception is controlled.

Frequently Asked Questions