Pregnancy Due Date Calculator

How the estimate is made

The standard method, Naegele's rule, adds 280 days to the first day of the last menstrual period. That assumes a 28-day cycle with ovulation on day 14, which is why the calculation counts from a period rather than from conception: the period date is known and the conception date usually is not. It also means the first two weeks of a 40-week pregnancy occur before conception, which is a persistent source of confusion.

Almost nobody delivers on the due date

Only about 4 percent of births occur on the estimated date. Around 60 percent occur within a week either side, and roughly 90 percent between 37 and 42 weeks. The due date is the midpoint of a distribution, not a deadline, and treating it as a fixed appointment causes a lot of unnecessary anxiety in the final fortnight.

Cycle length changes the answer

Naegele's rule assumes a 28-day cycle. Someone with a consistent 35-day cycle typically ovulates around day 21, so their due date is roughly a week later than the standard formula gives. Adjusting for known cycle length materially improves the estimate, and it is the most common reason a calculator and a clinic disagree.

Ultrasound dating is more accurate

A first-trimester ultrasound, ideally between 8 and 13 weeks, measures crown-rump length and estimates gestational age within about five days. Where it disagrees with the period-based date by more than about a week, clinical practice is to use the scan. Later scans are progressively less accurate for dating, because growth variation between individuals widens.

Terms worth knowing

Early term is 37 to 38 weeks and 6 days, full term 39 to 40 weeks and 6 days, late term 41 weeks, and post term 42 weeks and beyond. These definitions were revised because outcomes at 37 weeks are measurably different from those at 39, and the older single category of term obscured that.

This is general information and not medical advice. Your maternity care provider's dating supersedes any calculator, and any concern during pregnancy warrants contacting them directly.

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