How the due date is estimated
Pregnancy is counted from the first day of the last period, about two weeks before conception, and lasts on average 280 days (40 weeks) from then. This is Naegele’s rule, named after Franz Naegele, a German obstetrician who described it in the early nineteenth century: add a year, take away three months and add seven days.
This calculator adds 280 days, moved one day for each day your cycle is longer or shorter than 28. From conception it adds 266 days; from an IVF transfer, 261 days for a day-5 embryo and 263 for day 3.
| Starting from | Estimated due date |
|---|---|
| Last period 1 January 2026, 28-day cycle | 8 October 2026 |
| Last period 1 January 2026, 32-day cycle | 12 October 2026 |
| Conception 15 January 2026 | 8 October 2026 |
Only an estimate
Few babies arrive on their due date itself; most are born within about two weeks of it. Full term is counted from 37 weeks. Your midwife or doctor will usually confirm the date with an ultrasound dating scan, which can move it, and that date is the one to go by.
How to read the result
- Estimated due date: 40 weeks from the first day of the last period, moved for your cycle length.
- How far along today: in weeks and days, counted the way midwives and doctors count.
- Dates along the way: the weeks people often ask about, and full term from 37 weeks.
Common mistakes
- Counting from conception as if it were the period date: choose Conception instead.
- Forgetting to change the cycle length when cycles are longer or shorter than 28 days.
- Expecting the baby on the day: most arrive within about two weeks either side, and a scan may move the date.
How to use it
- Choose what you know: the first day of your last period, the date of conception or an IVF transfer.
- Pick the date, and your usual cycle length if it isn’t 28 days.
- Read your estimated due date, how far along you are today and the dates along the way.
Questions
How many weeks pregnant am I?
Counted from the first day of your last period, as midwives and doctors count. The calculator shows it in weeks and days for today.
Is the due date accurate?
It’s an estimate based on an average pregnancy. A dating scan gives a more reliable date, and most babies arrive in the weeks either side of it.
Is this medical advice?
No. It works out dates only. For anything about your pregnancy, talk to your midwife or doctor.
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