The estimated due date (EDD) is one of the first dates a mother-to-be learns. The most commonly used method for working it out is Naegele’s rule — a simple formula based on the date of the last menstrual period. Below we explain how it works, what its limitations are, and how to calculate an approximate date of birth yourself.
What is Naegele’s rule?
The rule developed by the German gynaecologist Franz Naegele assumes that pregnancy lasts on average 280 days, that is 40 weeks, counting from the first day of the last menstrual period (often abbreviated LMP). The starting point is not the moment of conception but rather the beginning of the last cycle, because a woman usually remembers this date more accurately.
The formula step by step
To work out the estimated due date, perform three operations on the date of the first day of the last menstrual period:
- Add 7 days to the day the last period started.
- Subtract 3 months.
- Add 1 year (when subtracting the months takes us back into the previous year).
In short: LMP + 7 days − 3 months. This is a simpler equivalent of adding a full 280 days and gives an identical result.
Worked example
Suppose the first day of the last menstrual period fell on 10 March 2026:
- Add 7 days: 10 March + 7 = 17 March 2026.
- Subtract 3 months: March − 3 = December, that is 17 December.
- Add 1 year, because we crossed back over the start of the year: 17 December 2026.
The estimated due date is therefore 17 December 2026. Remember that this is an approximate value — only about 4 percent of babies are born exactly on the predicted day.
Adjusting for an atypical cycle length
Naegele’s rule assumes a regular cycle lasting 28 days. If your cycle is longer or shorter, it is worth adjusting the date by the difference. For a 32-day cycle (4 days longer) you add 4 days to the result, and for a 25-day cycle you subtract 3 days. That is why, for women with irregular cycles, the date based on a first-trimester ultrasound is often more accurate.
Why is the date only approximate?
A birth is considered full-term between the 37th and 42nd week of pregnancy. This means that a completely normal birth can happen as much as three weeks before the predicted date or two weeks after it. The actual moment of birth is influenced by, among other things, cycle length, the course of the pregnancy and individual characteristics.
Ultrasound versus Naegele’s rule
In clinical practice, doctors often verify the date from Naegele’s rule with an ultrasound scan performed between the 11th and 14th week of pregnancy. Measuring the crown-rump length (CRL) of the foetus allows the gestational age to be estimated to within a few days. If the discrepancy between the two methods exceeds about a week, the date from the ultrasound usually takes precedence.
Naegele’s rule gives a quick, approximate answer, but it does not replace medical consultation or prenatal testing. The final due date and monitoring of the pregnancy are always determined by the doctor in charge of your care.
Work out your estimated due date: Due date calculator →