How Is a Pregnancy Due Date Calculated?
A due date is an estimate produced by a rule from the early 1800s. The Pregnancy Due Date Calculator applies it from the first day of the last period.
Naegele's rule
Take the first day of the last menstrual period, add one year, subtract three months, add seven days. That gives a date 280 days - forty weeks - after the start of the last period.
It is named after Franz Naegele, who popularised it in 1812, and it carries two assumptions: that the cycle is exactly 28 days, and that ovulation happens on day 14. Neither is true for most people. Cycles commonly run anywhere from 21 to 35 days, and ovulation timing varies within a cycle as well as between people.
So if your cycle is 32 days, the standard rule puts your date roughly four days early, and some calculators adjust for cycle length for exactly this reason.
Why so few babies arrive on it
Around four per cent of births happen on the estimated due date. About sixty per cent arrive within a week either side, and most of the rest within two.
That is not the rule failing. A due date is the midpoint of a distribution, not an appointment, and normal term spans 37 to 42 weeks. The useful way to read it is as the centre of a four-week window in which a healthy birth is entirely expected.
First pregnancies tend to run slightly longer than later ones, by a few days on average.
Why an early scan is more accurate
A dating ultrasound in the first trimester measures the embryo directly, and early growth is consistent enough between pregnancies that the measurement is a better predictor than the date of a period - particularly when cycles are irregular or the last period's date is uncertain.
Where a scan and the calculation disagree by more than about a week, the scan is generally used. That is standard practice rather than a judgement about either method, and it is why the date you are given at a first appointment sometimes moves.
This is also the point at which the honest answer is that a calculator is for curiosity and planning, and your midwife or doctor is for anything that matters. Dating affects decisions about monitoring and induction, and those belong with the people who can see your whole history.
Common questions
Can I work it out from conception instead?
If you know the conception date, add 266 days rather than 280 - the fourteen-day difference is the assumed gap between period and ovulation. The Ovulation Calculator estimates that timing.
What are the trimesters?
Roughly weeks 1 to 13, 14 to 27, and 28 to birth. The boundaries are conventions and vary a little between sources.
Why is pregnancy counted from the last period?
Because it is the one date most people can identify reliably. It means the first two weeks of a "pregnancy" precede conception.
Does the date change if the baby is measuring large or small?
Later scans do not usually change the date, because growth varies much more by then.
Curious what the rule gives?