Due Date Calculator
Pregnancy due date from your last period, conception, IVF transfer or a scan.
How to use Due Date Calculator
- Choose what you know: last period, conception date, IVF transfer, a scan result or the due date itself.
- Enter the date (and your cycle length, embryo age or the weeks measured at the scan).
- Read the estimated due date, how far along you are today and the key dates ahead.
How it works
From the first day of your last period, the due date is 280 days (40 weeks) later, known as Naegele’s rule, moved later or earlier by one day for each day your cycle is longer or shorter than 28. From a known conception or ovulation date it is 266 days later. For IVF it is 266 days minus the embryo’s age from the transfer date: 261 days after a day-5 transfer, 263 after a day-3 transfer. From an ultrasound, it is the scan date plus the days remaining to 40 weeks at the measured age.
Weeks of pregnancy are counted the way clinicians count them, from the first day of the last period (about two weeks before conception). Trimesters follow the usual convention: first to 13 weeks + 6 days, second from 14 weeks, third from 28 weeks. Dates are calculated in whole calendar days, so daylight-saving changes do not shift them.
Example
Last period started 1 January 2026 with a 28-day cycle: 280 days later is 8 October 2026. Conception was around 15 January. On 12 March you would be 10 weeks 0 days pregnant.
Frequently asked questions
How accurate is a due date?
It is an estimate of the middle of a range. Only about 4–5% of babies are born on the due date itself; most arrive within two weeks either side. A first-trimester ultrasound is the most accurate way to date a pregnancy, and your care team may change the date after it.
How is the due date calculated for IVF?
From the transfer date: add 261 days for a day-5 embryo (blastocyst) or 263 days for a day-3 embryo. That equals 266 days from fertilisation.
Why does pregnancy count from the last period and not conception?
Because the first day of the last period is usually known and conception usually is not. It means you are counted as about two weeks pregnant at conception.
What if my cycles are irregular?
The last-period method becomes unreliable. Use the conception date if you know it, or wait for a dating scan.
Is my data stored?
No. The dates stay in your browser and are not sent or saved anywhere.
Limitations
- An estimate only. It does not replace dating by ultrasound or advice from your midwife or doctor.
- The last-period method assumes ovulation 14 days before the next period.
- Twins and other multiples are usually born earlier than the 40-week date.
Last reviewed November 9, 2026. How we build and test tools.