CalcVitalsDue Date Calculator — Pregnancy
Due dates are estimated with Naegele's rule: 280 days — 40 weeks — from the first day of your last period. A last period starting January 1, 2026 gives an estimated due date of October 8, 2026. Only about 1 baby in 20 arrives on the exact day; the surrounding weeks are the real window.
How it works
Naegele's rule is two centuries old and still the standard first estimate: take the first day of the last menstrual period, add seven days, subtract three months, add a year — which is the same as adding 280 days. It assumes a 28-day cycle with conception near day 14, so this page adjusts for your actual cycle length: longer cycles shift the estimate later, shorter ones earlier, day for day.
Hold the result the way midwives do — as the centre of a window, not an appointment. A full-term pregnancy runs 37 to 42 weeks, and only around 5% of babies are born on their estimated date. The date's real jobs are administrative and medical: scheduling scans, timing screening tests, and deciding when a pregnancy running long needs monitoring or induction.
If you know a conception date (from fertility treatment or precise tracking), count 266 days from it instead — the conception calculator on this site does exactly that, in both directions. And if periods are irregular or the LMP date is uncertain, an early ultrasound measures the embryo directly and beats every calendar method, this one included.
The method
Naegele's rule: due date = LMP + 280 days, adjusted by (cycle length − 28) days. Conception estimate = LMP + 14 + adjustment. Trimester boundaries at 14 and 28 completed weeks; full-term window 37–42 weeks per ACOG definitions.
Frequently asked questions
How accurate is this due date?
It is an estimate with a five-week full-term window around it. Roughly 80% of babies arrive within two weeks either side of the date. First-trimester ultrasound is the most accurate dating method and overrides the calendar when they disagree.
My cycle is irregular. Can I still use this?
You can get a rough figure, but the day-14 ovulation assumption is exactly what irregular cycles break. Enter your most typical cycle length, and treat an ultrasound date as authoritative when you have one.
Why 280 days when pregnancy is '9 months'?
Forty weeks is 280 days — nine months plus about a week, because calendar months are longer than four weeks. The count starts at the last period, roughly two weeks before conception, which is why 'weeks pregnant' always runs two weeks ahead of the embryo's actual age.