How the date is calculated
The standard method is Naegele's rule: take the first day of the last menstrual period, add one year, subtract three months, and add seven days — which works out to 280 days, or 40 weeks, from the last period. It dates from the early nineteenth century and remains the basis of most estimates.
Its central assumption is a regular 28-day cycle with ovulation on day 14. Cycle length varies considerably between people and between cycles, and ovulation timing varies even more — studies tracking ovulation directly find it occurs across a wide window rather than reliably at mid-cycle. For someone with a 35-day cycle, the rule can be off by a week before anything else is considered.
Because pregnancy is dated from the last period rather than from conception, the first two weeks of a "pregnancy" precede conception entirely. Gestational age is therefore about two weeks greater than the actual age of the embryo, which is a persistent source of confusion.
How likely the date is to be right
Very few babies arrive on their due date — around 4 percent, which is close to what chance alone would give across a spread of plausible days. Roughly 80 percent of spontaneous births occur between 37 and 42 weeks, and term itself is now subdivided: early term at 37 to 38 weeks, full term at 39 to 40, late term at 41, and post-term beyond 42.
First pregnancies tend to go slightly longer than subsequent ones. Population studies also find modest differences by maternal age and ethnicity, and there is meaningful natural variation in gestation length that no dating method captures.
An early ultrasound is substantially more accurate than a last-period calculation. A crown-rump length measurement in the first trimester dates the pregnancy to within about five days, and clinical guidance is to revise the estimated date when ultrasound dating differs from last-period dating by more than a threshold — commonly five to seven days in the first trimester. Later scans are progressively less accurate for dating, because growth variation widens.
What the estimate is for
The date matters clinically less as a prediction than as a reference. It determines when screening tests are offered, when a pregnancy is considered pre-term or post-term, when growth is assessed against expected ranges, and when induction may be discussed — all of which depend on knowing gestational age rather than on the baby arriving on schedule.
Treating it as an appointment causes avoidable distress. A more realistic framing is a two-week window around the date, and communicating a range rather than a day to family and employers avoids the daily questions that follow a passed due date.
This calculator applies Naegele's rule to the information you enter. It is not a medical assessment and cannot account for cycle irregularity, known conception date, assisted reproduction — where the transfer date gives far more accurate dating — or any individual clinical factor. Your midwife, obstetrician or doctor holds the authoritative date, normally based on your dating scan, and any concern during pregnancy should go to them rather than to a calculator.