Pregnancy due date calculator
Estimate the due date from the first day of your last period, from a known conception date, or from an IVF embryo transfer. The calculator uses Naegele’s rule, adjusts for a cycle that is not 28 days, and shows how many weeks and days pregnant you are today along with the dates of the main milestones. It is an estimate, not a medical assessment — a dating scan is what your maternity team will rely on.
How to use the Due Date Calculator
- Choose what you are calculating from.
- Enter that date as YYYY-MM-DD.
- If you are using your last period and your cycle is not 28 days, set the cycle length.
- Press Estimate due date to see the date, the current week and the milestones.
Naegele’s rule
Pregnancy is dated from the first day of the last menstrual period, not from conception, and a full-term pregnancy is counted as 280 days — 40 weeks — from that day. Naegele’s rule is the shorthand: take the first day of the last period, add one year, subtract three months and add seven days. Adding 280 days gives the same answer and is easier to be exact about.
Worked example: a last period starting 1 June 2026 gives an estimated date of delivery of 8 March 2027, with conception around 15 June 2026.
Adjusting for cycle length
The 280-day figure assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, ovulation is later and the due date moves back by the difference: EDD = LMP + 280 + (cycle length − 28). A 32-day cycle pushes the estimate four days later; a 24-day cycle brings it four days earlier. If you know the conception date, 266 days is added to that instead, and no cycle adjustment is needed. For IVF the dating is more precise still, because the embryo's age is known: a day-5 transfer is 261 days before the due date and a day-3 transfer 263 days.
The milestones
| Stage | Weeks |
|---|---|
| End of the first trimester | 13 weeks 6 days |
| Anomaly scan window (UK) | 18 to 20 weeks 6 days |
| End of the second trimester | 27 weeks 6 days |
| Early term | 37 weeks 0 days |
| Full term | 39 weeks 0 days |
| Late term | 41 weeks 0 days |
| Post term | 42 weeks 0 days |
Gestational age is written as weeks and days — "32+4" means 32 weeks and 4 days. Note that the first two weeks of that count happen before conception, which is why the "weeks pregnant" number is roughly two weeks ahead of the age of the pregnancy itself.
How accurate is a due date?
It is an estimate of the middle of a range, not an appointment. Babies arrive across a spread of weeks either side, and dating by last period is less reliable when cycles are irregular, when the date of the last period is uncertain, or when there was bleeding in early pregnancy. An ultrasound scan in the first trimester dates a pregnancy more accurately than a menstrual date, and UK maternity services will revise the due date from that scan if the two differ by more than a few days.
Medical disclaimer. This calculator provides general information only. It is not medical advice, it cannot confirm a pregnancy or its progress, and it must not be used for decisions about care, medication or travel. Speak to a midwife, GP or obstetrician, and seek urgent medical attention for any bleeding, pain, reduced fetal movement or other concern.
Frequently asked questions
How is a due date calculated?
By adding 280 days to the first day of the last menstrual period, adjusted for cycle length. From a known conception date, 266 days are added instead.
Why is pregnancy dated from the last period?
Because that date is usually known, while the moment of conception rarely is. It means the first two weeks of the 40 are counted before conception happens.
What if my cycle is not 28 days?
The estimate shifts by the difference. A 32-day cycle moves the due date four days later, because ovulation happens four days later than the standard assumption.
Is a scan more accurate than this?
Yes. A first-trimester ultrasound dates a pregnancy more accurately than a menstrual date, and maternity services will usually revise the due date to match the scan.
Privacy
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Last updated 2026-09-23.