Due Date Calculator

How your due date is calculated (Naegele's rule)

The standard method obstetricians use is Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). It assumes a textbook 28-day cycle with ovulation on day 14 — so if your cycles run longer or shorter, the calculator above shifts the estimate by the same number of days your cycle differs from 28, since a longer cycle means later ovulation and a later due date.

ACOG (the American College of Obstetricians and Gynecologists) still recommends LMP-based dating as the default, confirmed or adjusted by a first-trimester ultrasound when the two differ by more than about a week.

Due date from a known conception date

If you know your conception date directly (from ovulation tracking or a single-cycle attempt), the calculator adds 266 days (38 weeks) instead of 280 — the difference is simply the roughly two weeks between LMP and ovulation that the LMP method builds in automatically.

IVF due date: day 3 vs. day 5 transfers

For IVF pregnancies the embryo's exact age is known at transfer, so the math is more precise than LMP dating. The rule is 266 days minus the embryo's age at transfer: a day-5 blastocyst transfer adds 261 days, and a day-3 cleavage-stage transfer adds 263 days, because the embryo was already 5 or 3 days into development before it was placed.

This is the convention used by fertility clinics and IVF due-date tools, and it's why an IVF due date is usually a firmer estimate than an LMP-based one.

Trimesters and what happens when

Pregnancy is divided into three trimesters: the first runs weeks 1–13, the second weeks 14–27, and the third weeks 28 through birth (around week 40). The calculator shows which trimester you're in today and the calendar date each one begins, plus a handful of common milestones — end of first trimester, the anatomy-scan window around week 18–20, the viability milestone around week 24, and full term at week 39.

How accurate is a due date, really?

Only about 1 in 20 babies are born on their exact estimated due date. Full-term birth is normally defined as anywhere from 37 to 42 weeks, so the date from this calculator — or any due-date method — is a statistical midpoint, not a deadline. It's still the number doctors use to schedule prenatal screening and monitor growth against expected norms.

What this calculator does not do

This tool estimates dates from the information you enter using standard obstetric formulas — it does not replace a dating ultrasound, blood test, or your prenatal care provider, and it cannot detect pregnancy complications. If your cycles are irregular, use the conception-date or IVF mode if you have one of those dates, or ask your provider for an ultrasound-based estimate.