Pregnancy Due Date Calculator

Three methods to compute an Estimated Date of Delivery (EDD): LMP uses Naegele's rule (LMP + 280 days) with optional cycle-length adjustment. Conception counts 266 days from conception. Ultrasound back-calculates LMP from the scan date and reported gestational age, then projects forward 280 days. Shows current weeks/days along, trimester (1: 0–12w, 2: 13–26w, 3: 27w+), and days remaining or overdue.

Gestational age is counted from the last menstrual period rather than from conception, which puts someone at four weeks pregnant about when a test first turns positive and roughly two weeks before the pregnancy began in any biological sense. The two-week offset is why the LMP and conception methods differ by fourteen days, and why a due date can look wrong to anyone reasoning from the date of conception instead.

Naegele's rule assumes a 28-day cycle with ovulation on day 14, which is a convenient average rather than a common case. Cycles vary between people and between months for the same person, and it is the follicular phase before ovulation that stretches or shortens while the luteal phase stays near fourteen days. The cycle-length adjustment shifts the estimate accordingly, and for an irregular cycle the LMP method is the weakest of the three.

An early ultrasound is the most reliable input here, and the reason is that early embryos grow at nearly the same rate. A crown-rump length in the first trimester dates a pregnancy to within about five days; by the third trimester the same measurement carries an error of a couple of weeks, because genuine variation in size has taken over. Clinical practice is to set the due date from the earliest good scan and not revise it later.

A due date is a midpoint, not an appointment. Only a few percent of births happen on it, and the normal range spans several weeks either side. This calculator is arithmetic for planning; the date that matters is the one in your medical record, set by the people providing your care.