Pregnancy Due Date Calculator

How this due date is calculated

Enter the first day of your last menstrual period (LMP). Your due date is that date plus 280 days (40 weeks), using Naegele’s rule, adjusted for your cycle length if it differs from 28 days.

Naegele’s rule dates from 1812, when German obstetrician Franz Naegele observed that pregnancies anchored to a known last period averaged 280 days. It still works because human gestation is remarkably consistent once you fix that anchor. The only real assumption is a 28-day cycle with ovulation near day 14 — which is exactly why this site separates cycle-length and luteal-phase adjustments onto their own pages instead of baking one average into every result.

What to expect

This is the standard due date calculator — the one most people mean when they ask "when is my baby due?" Enter the first day of your last menstrual period and the tool adds 280 days (40 weeks) using Naegele's rule, the same method your doctor or midwife will try first. One entry returns three useful answers at once: your estimated due date, the estimated conception date (about two weeks after the first day of your last period), and roughly how many weeks and days you are today.

Naegele's rule has been the obstetric default for two centuries because human gestation is remarkably consistent once you anchor it to a known date. The rule assumes a 28-day cycle with ovulation around day 14. If your cycles are reliably longer or shorter than 28 days, the due date shifts by the difference — the cycle-length calculator on this site applies that adjustment in one step, and the luteal-phase calculator fine-tunes it further if you track the post-ovulation window.

Every due date is an estimate with a built-in window. Only about 4% of babies arrive exactly on the predicted day; most arrive within a week or two of it, either side. That is why the date is best used for planning — scheduling appointments, telling family, ordering the car seat — rather than as a deadline. Your provider will confirm or adjust the date after an early ultrasound, which is the most accurate dating method in pregnancy because early fetal size follows a very predictable curve.

Not sure this is the right calculator for your situation? Pick the method that matches what you actually know: if you tracked ovulation or conception, use the conception-date calculator; if your pregnancy started with an embryo transfer, the IVF calculator covers Day-3, Day-5 and Day-6 transfers plus frozen, donor and surrogate cycles; if you already have a scan report with a gestational age, the ultrasound calculator works backwards from it; and if you are expecting twins, the twins page shows the official date alongside the typical earlier-arrival window.

Once you have a due date you like, the "How far along am I?" tool turns it into your current week and day, your three trimester date ranges, and a countdown of days remaining — and the week-by-week page lays out the milestones ahead, from the first scan to full term.

Important note

These calculators use standard formulas — Naegele's rule (LMP + 280 days) and common IVF conventions (transfer date + 260–263 days). They give estimates for planning only. They are not a substitute for prenatal care, and only your provider can confirm your due date and track your baby's development. See the disclaimer for details.

FAQ

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