Due Date Calculator
Estimate your baby’s due date from your last period, conception date or IVF transfer — with your current gestational age and how far along you are.
How it works
Naegele’s rule: due date = LMP + 280 days, shifted by (cycle length − 28) · from a conception date, add 266 days · IVF: day-3 transfer + 263, day-5 transfer + 261.
Pregnancy is dated as 40 weeks (280 days) from the first day of your last menstrual period, which is roughly two weeks before conception — so “40 weeks pregnant” includes about two weeks before you actually conceived. Naegele’s rule assumes ovulation on day 14 of a 28-day cycle; a longer or shorter cycle shifts the estimate, and the conception and IVF methods sidestep the cycle by counting from a known date. An early ultrasound is the most accurate way to date a pregnancy, so treat this as a starting estimate your provider can refine.
Worked example
A last period starting January 1, 2026 with a 28-day cycle: adding 280 days (40 weeks) lands on October 8, 2026 — the estimated due date, with conception estimated around January 15.
Frequently asked questions
How accurate is a due date?
It’s an estimate: only about 4% of babies are born on their due date, and most arrive within a week or so either side. An ultrasound before about 13 weeks dates a pregnancy most accurately — always use the date your doctor or midwife gives.
Why 40 weeks when pregnancy is “nine months”?
The 280 days are counted from the first day of your last period, about two weeks before conception, so the count starts before you were actually pregnant. Measured from conception, gestation is closer to 38 weeks.
Does my cycle length really change the date?
Yes, a little. The standard formula assumes a 28-day cycle with ovulation on day 14. A longer cycle pushes ovulation — and the due date — later, and a shorter one pulls it earlier. Only a scan can confirm it, so ask your provider.
Related calculators
Estimates based on standard formulas and average cycles — every pregnancy is different. This is not medical advice; your doctor or midwife, with an ultrasound, gives the accurate dates and guidance.