Pregnancy Due Date FAQ

Due date calculators give a statistical estimate, not a guarantee. They use standard obstetric formulas like Naegele’s Rule (LMP + 280 days) and are accurate to within about one to two weeks for most individuals. Only about 5% of babies are born on their exact due date, and first-trimester ultrasound measurements are generally more precise than a calendar-based calculation alone.

LMP dating counts from the first day of your last menstrual period and assumes ovulation around day 14, so it adds 280 days (40 weeks). Conception dating counts from the actual fertilization date and adds 266 days (38 weeks). The two methods produce the same due date when ovulation happened exactly 14 days after the LMP. If your cycle is longer or shorter than 28 days, conception dating or an early ultrasound may be more accurate.

IVF due dates are based on the embryo transfer date and the age of the embryo that was transferred. For a day-5 blastocyst transfer, 261 days are added; for a day-3 cleavage transfer, 263 days are added. Because the embryo’s age is known precisely, IVF dating removes the assumptions about ovulation that LMP-based calculations rely on.

Yes. If a first-trimester ultrasound (especially between 8 and 14 weeks) shows your baby measuring ahead of or behind the calendar-based estimate, your provider may adjust your official due date. Dating becomes less accurate later in pregnancy, because second- and third-trimester measurements reflect growth patterns rather than gestational age.

The third trimester is generally considered to begin at 27 weeks of gestation and lasts until birth. The first trimester runs from week 0 through week 12, the second from week 13 through week 26, and the third from week 27 onward.