Due Date Calculator

Convention and certainty: most people know their LMP date but not the conception date, so medicine standardized on LMP + 280 days (Naegele's rule) — which is also why you're 'four weeks pregnant' barely two weeks after conception. The cycle-length adjustment matters because ovulation shifts later in longer cycles.

It's a center of a range, not a deadline: only about 4–5% of births happen on the due date, and the large majority fall within two weeks either side. A first-trimester ultrasound measures the embryo directly and is more accurate than any date arithmetic — providers routinely re-date pregnancies after one, and that date wins.

IVF is the one case where the timeline is truly known — the embryo's age at transfer is documented. A day-5 blastocyst transfer counts 261 days forward, a day-3 transfer 263, with no cycle guesswork at all. It's the most precise dating any pregnancy gets.

Working boundaries rather than biological switches: the second trimester at 14 weeks (when many people feel better and risk drops), the third at 28, full term at 39 — the point research shows outcomes are best, which is why elective deliveries aren't scheduled earlier. Use them for planning; your provider's dating governs decisions.