Due Date Calculator

It depends which date you enter. From your last menstrual period (LMP), the calculator adds 280 days — this is Naegele's Rule, recognized by ACOG. From a known conception date, it adds 266 days instead, since conception happens roughly two weeks after LMP. From an IVF transfer date, it adds 263 days for a Day 3 embryo or 261 days for a Day 5 embryo, since the embryo's age is already known at transfer.

It's an estimate, not a prediction — ACOG notes only about 4 in 100 babies are born on their exact due date, though most arrive within roughly two weeks either side. A first-trimester ultrasound measuring crown-rump length dates a pregnancy more precisely than LMP-based math. If your provider's scan date and this estimate differ by more than about a week, ACOG guidance is to go with the scan date.

Naegele's Rule assumes a 28-day cycle with ovulation on day 14, so this calculator shifts the estimate to match your actual cycle length — a 35-day cycle moves the due date about 7 days later than the default math would give. If your cycles are irregular or you're unsure of your cycle length, LMP-based dating becomes less reliable, and an early ultrasound is the better source of truth.

With IVF, the embryo's exact age is known at transfer, so there's no need to estimate ovulation timing the way LMP dating does. A Day 3 (cleavage-stage) transfer adds 263 days to reach a 280-day pregnancy; a Day 5 (blastocyst) transfer adds 261 days, since the embryo is already 2 days older at transfer.

These are ACOG/SMFM-defined gestational-age windows within the third trimester: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days (associated with the best average outcomes), late term is 41 weeks 0 days to 41 weeks 6 days, and postterm is 42 weeks or beyond, which providers typically monitor more closely.