Pregnancy due date calculator
Start with whatever you know. Get your estimated due date, exactly how far along you are, and every date that matters, with the arithmetic shown.
Six ways to date a pregnancy
Each method is simple date arithmetic. What differs is how certain the starting point is.
- Last periodFirst day + 280 days
Plus or minus the difference between your cycle and 28 days.1,2
- ConceptionConception + 266 days
38 weeks after fertilization, which is 40 weeks of pregnancy.1
- IVF transferTransfer + 266 − embryo age
263 days after a day-3 transfer, 261 after a day-5 transfer.1
- UltrasoundScan date + 280 − age at scan
Or convert a crown-rump length with the Robinson equation.1,3
- Known due dateWeeks pregnant = 280 − days left
Your clinician’s date is kept exactly as given.1
- Weeks pregnantDate + 280 − weeks and days
Useful when you know how far along you are today.1
Clinicians date a pregnancy once, as early as possible: by IVF dates for IVF pregnancies, otherwise by the last period confirmed or corrected by an early ultrasound.1 How due dates are calculated
A due date is the middle of a window
Your due date marks 40 weeks 0 days. It is a planning anchor, not a prediction. In an Australian study of 18,708 single births after spontaneous labor, about 5% happened on the estimated due date and about two-thirds within seven days of it.4
Even when ovulation is measured precisely, natural pregnancy length varies by weeks between people.5 That is why care teams talk about term windows. A baby born from 39 weeks 0 days to 40 weeks 6 days is full term; 37 and 38 weeks are early term, and 41 weeks is late term.6
| Completed weeks | Share of spontaneous labor onsets |
|---|---|
| Before 37 weeksPreterm | 7.7% |
| 37 weeksEarly term | 5.1% |
| 38 weeksEarly term | 12.1% |
| 39 weeksFull term | 25.4% |
| 40 weeksFull term | 32.5% |
| 41 weeksLate term | 16.2% |
| 42 weeks or morePostterm | 0.9% |
Questions people ask
How is a due date calculated?
Most due dates are 280 days (40 weeks) after the first day of your last period, adjusted if your cycle is usually longer or shorter than 28 days. If you know when you conceived, it is 266 days after conception. Early ultrasound dating is the most accurate method and often confirms or replaces the period-based date.1
Why does pregnancy count from my last period, before I was pregnant?
Because the first day of the last period is usually known and ovulation is not. Counting from it means you are already about 2 weeks “pregnant” at conception, and a pregnancy lasting 40 weeks by this count is about 38 weeks from conception.1,8
How many babies are born on their due date?
Few. In a large Australian study of births after spontaneous labor, about 5% were born on the estimated due date and about two-thirds within a week either side of it.4
Can my due date change?
Yes, usually once, early. If a first-trimester ultrasound differs from your period-based date by more than set thresholds, your care team may change your due date. Once set from an early scan, it should rarely change based on later scans.1
What do I put if my cycles are irregular?
Choose “Not sure or irregular” and the calculator assumes 28 days, which may be inaccurate for you. An early ultrasound is the more reliable way to date a pregnancy when cycles are irregular or the last period date is uncertain.1
Is my information saved or sent anywhere?
No. The calculation runs in your browser. We do not receive your dates, and nothing is stored after you close the page unless you copy a share link or download a calendar file yourself.
Sources
- Committee Opinion No. 700: Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists, with AIUM and SMFM, 2017
- Naegele’s rule: a reappraisalBJOG 107(11):1433–1435, 2000
- A critical evaluation of sonar “crown-rump length” measurementsBritish Journal of Obstetrics and Gynaecology 82(9):702–710, 1975
- Predicting date of birth and examining the best time to date a pregnancyInternational Journal of Gynecology & Obstetrics 123(2):105–109, 2013
- Length of human pregnancy and contributors to its natural variationHuman Reproduction 28(10):2848–2855, 2013
- Committee Opinion No. 579: Definition of Term PregnancyAmerican College of Obstetricians and Gynecologists, 2013
- Inducing labour (NG207), table 1National Institute for Health and Care Excellence, 2021
- How Your Fetus Grows During PregnancyAmerican College of Obstetricians and Gynecologists
Links checked November 12, 2026. Citing a source does not mean it endorses this site.