Pregnancy Due Date Calculator

The standard due date calculation is Naegele's rule, published in the early nineteenth century: take the first day of the last menstrual period, add 280 days, and that is your estimate. It survives because it is simple and because 280 days is a reasonable average, not because it is precise. It embeds two assumptions that are frequently wrong — that your cycle is twenty-eight days long, and that you ovulated on day fourteen of it.

This calculator lets you work around both. If you know your cycle length, the due date shifts to match, since someone with a thirty-five day cycle typically ovulates a week later than the rule assumes and would otherwise be dated a week early. If you know your conception date, or had an IVF transfer where the embryo's age is known exactly, those paths skip the guesswork entirely and are correspondingly more accurate.

What no date-based method can do is account for individual variation in gestation length, which is genuinely large. Studies tracking ovulation directly have found natural pregnancy length varying by around five weeks between individuals even after removing dating error. This is why an early ultrasound overrides the calculation in clinical practice, and why the term window spans from thirty-seven to forty-two weeks rather than fixing on a single day.