Naegele's rule
The standard clinical method dates from Franz Naegele in the early 1800s and is still what most providers use as a starting point. It counts 280 days — 40 weeks — from the first day of your last menstrual period.
- 280 days
- 40 weeks, the conventional full-term length
- LMP
- First day of the last menstrual period, not the last day
The equivalent shortcut: take the LMP, subtract three months, add seven days, and add a year.
How accurate is a due date?
Only about 4% of babies arrive on their estimated due date. Roughly 80% are born within two weeks either side. The due date is the midpoint of a distribution, not an appointment.
| Timing | Share of births | Classification |
|---|---|---|
| Before 37 weeks | ~10% | Preterm |
| 37–38 weeks | ~26% | Early term |
| 39–40 weeks | ~38% | Full term |
| 41 weeks | ~19% | Late term |
| 42 weeks and beyond | ~7% | Post term |
Naegele's rule also assumes a 28-day cycle with ovulation on day 14. If your cycles run 32 days, ovulation is later and the due date shifts about four days back; a 24-day cycle shifts it four days forward. This calculator adjusts for your cycle length.
IVF and conception dating
With IVF the conception date is known precisely, so the estimate is far more reliable. The calculation works backwards from the transfer:
Day 3 transfer: due date = transfer date + 263 days
The logic is that a full-term pregnancy is 266 days from conception, and the embryo is already three or five days old at transfer.
For a natural conception where you know the date — from ovulation tracking or a single act of intercourse — the calculation is the conception date plus 266 days. This is more accurate than the LMP method if your cycles are irregular.
To track progress week by week once you have a due date, use the pregnancy calculator.
Frequently asked questions
How is a due date calculated?
Naegele's rule adds 280 days — 40 weeks — to the first day of your last menstrual period. It assumes a 28-day cycle with ovulation on day 14.
If your cycles are longer or shorter, the due date shifts accordingly. This calculator adjusts for cycle length.
How accurate is my due date?
Only about 4% of babies are born on it. Around 80% arrive within two weeks either side, which is why it is called an estimate.
A first-trimester ultrasound is considerably more accurate, dating the pregnancy within about five days.
Why is pregnancy counted from my last period?
Because the LMP is a date most people can identify, while the moment of conception usually is not. It means the first two weeks of a "40-week" pregnancy occur before conception.
It is a clinical convention chosen for reliability, not a biological statement.
How does a longer or shorter cycle change the date?
Naegele's rule assumes ovulation on day 14. With a 32-day cycle you likely ovulate around day 18, pushing the due date about four days later. A 24-day cycle brings it forward by about four days.
Enter your average cycle length above and the calculator adjusts.
Can the due date change during pregnancy?
Yes. If an early ultrasound differs from the LMP-based date by more than five to seven days, providers usually revise the due date to match the scan.
Later scans are less reliable for dating, because growth varies more between babies as pregnancy progresses.