Due date calculator
Naegele's rule from your last period, with a cycle-length adjustment — or from conception or IVF transfer.
Enter a date to see an estimated due date.
How a due date is worked out
A due date is 280 days — 40 weeks — from the first day of your last menstrual period. That rule is named after Franz Naegele, who set it out in 1812, and it is still what almost every calculator and clinic uses.
It contains an assumption worth knowing about: that your cycle is 28 days and that you ovulated on day 14. Pregnancy actually lasts about 266 days from conception, and the extra fortnight is the gap between your last period and ovulation. If your cycles are longer or shorter than 28 days, the estimate shifts by the difference, which is why this calculator asks.
Formulas
Worked examples
| Method | Due date from 1 January 2026 |
|---|---|
| Last period, 28-day cycle | 8 October 2026 |
| Last period, 35-day cycle | 15 October 2026 |
| Last period, 21-day cycle | 1 October 2026 |
| Conception on that date | 24 September 2026 |
| Day-5 IVF transfer on that date | 19 September 2026 |
| Day-3 IVF transfer on that date | 21 September 2026 |
A seven-day difference in cycle length moves the estimate by a full week — the single largest source of variation in an LMP-based date.
What the weeks mean
| Stage | Gestational age | From a 1 January period |
|---|---|---|
| Early term | 37 weeks 0 days | 17 September 2026 |
| Full term | 39 weeks 0 days | 22 October 2026 |
| Estimated due date | 40 weeks 0 days | 8 October 2026 |
| Late term | 41 weeks 0 days | 1 October 2026 |
| Post term | 42 weeks 0 days | 22 October 2026 |
The due date is the midpoint of a range, not an appointment. Most of that range is entirely normal.
Worth knowing
- About 4% of babies arrive on their due date. A normal term delivery is anywhere from 37 to 42 weeks, which is a five-week window with the due date sitting three weeks from its start.
- A first-trimester ultrasound dates a pregnancy more accurately than any calculation, because early embryos grow at a very consistent rate. Where a scan and an LMP date disagree, your care provider will use the scan.
- Gestational age is counted from the last period, not from conception, so at "8 weeks pregnant" the embryo is about 6 weeks old. This confuses nearly everybody at first and it is not a mistake in the arithmetic.
- Naegele's original rule is often taught as "subtract three months, add seven days, add a year". That mnemonic and the 280-day count are not the same function — month lengths and leap years make them differ by a day or two for most start dates.
Frequently asked questions
- How accurate is a due date calculator?
- It gives you the midpoint of a five-week window, and only about 4 per cent of babies arrive on the day itself. Roughly half of first-time mothers deliver within a week either side. An early ultrasound narrows the estimate considerably; a calculation from the last period cannot.
- Why is pregnancy counted as 40 weeks when it lasts 38?
- Because the count starts at the first day of your last period, which is about two weeks before you conceive. Nobody usually knows the day of conception, but most people know when their last period started, so the convention trades accuracy for something you can actually answer.
- What if my cycle is not 28 days?
- The estimate shifts by the difference, because Naegele's rule assumes ovulation on day 14. On a 35-day cycle you ovulate around day 21, so the due date moves a week later. This calculator applies that adjustment; many do not, which is one reason two sites can disagree.
- How is an IVF due date different?
- It is more precise, because the date of fertilisation is known rather than inferred. The due date is 266 days from fertilisation, minus however many days the embryo had already developed before transfer — so a day-5 blastocyst transfer adds 261 days, and a day-3 embryo transfer adds 263.