Due Date Calculator

This calculator dates a pregnancy from one of two starting points and projects everything else by fixed day counts. From the last menstrual period it applies Naegele's rule — 280 days (40 weeks) — and also back-fills an estimated conception date 14 days after the LMP. From a known conception date it adds 266 days (38 weeks) instead, and the two methods agree when a 28-day cycle is assumed: conception is exactly 14 days after the period that precedes it, and 280 minus 14 is 266. The method switch changes more than the anchor — it changes what the conception output means: under LMP it is an estimate derived from the 14-day assumption, under Conception it simply echoes your input.

Current week, trimester, days remaining, and percent complete are all computed against today, so they move on their own as the calendar advances and read their capped values (week 40-plus, trimester 3, zero days, 100 percent) once a due date has passed. Trimester boundaries are weeks 12 and 27 in this engine — the first trimester ends at week 12, the third begins at week 28. The 40-week figure is a statistical midpoint, not a deadline: only a small minority of births arrive on the calculated date, and the model assumes the textbook cycle regardless of your actual cycle length. Naegele's rule itself dates to the early 1800s and remains the standard obstetric dating convention today, though a widely cited historical reappraisal (Baskett & Nagele, 2000, BJOG) argues the rule, as commonly applied, rounds a day or two earlier than its originator intended — a small but real source of the gap between the calculated date and actual delivery, on top of natural biological variation in gestation length.