A pregnancy due date is estimated with Naegele's rule, a method dating back to the early 1800s that is still the standard starting point worldwide: take the first day of the last menstrual period (LMP), subtract three months, add seven days, and add one year. The rule assumes a regular 28-day cycle with ovulation on day 14 and a gestation of 280 days, or 40 weeks, from the LMP. Because ovulation actually happens about two weeks after the LMP, the embryo is only about 38 weeks old at the '40-week' due date — the two-week offset is baked into the convention, which is why pregnancy is dated from the period rather than from conception.
Treat the date as the middle of a window, not an appointment: only about 5 percent of babies arrive on their due date, with most births falling within the two weeks before or after. First-trimester ultrasound dating, which measures the embryo when growth is most uniform, is more accurate than LMP dating and is what clinicians prefer when the two disagree by more than a few days. Irregular cycles, recent hormonal contraception, or uncertainty about the LMP date all reduce the rule's reliability — in those cases the dating scan, not the calendar math, sets the official date.
The due date anchors the whole pregnancy timeline: the first trimester runs to week 13, the second to week 27, and the third to delivery, with key milestones like the anatomy scan around 18 to 22 weeks and viability around 24 weeks counted from the same starting point. This calculator applies Naegele's rule to your LMP for an informational estimate. It cannot replace clinical dating — confirm the date with your midwife or doctor, especially if your cycles are irregular, and seek prompt care for any concerning symptoms rather than consulting a calculator.