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How Your Pregnancy Due Date Is Calculated
A due date is estimated from your last period, not conception — which is why the count is 40 weeks. Learn how the estimate works, why cycle length matters, and how accurate the date really is.
Updated 25 August 2026 · 2 min read
The estimated due date is one of the first numbers you get in pregnancy, and the way it's worked out surprises many people: it's counted from your last period, not from conception. That quirk explains the famous "40 weeks" and why the count starts before you were even pregnant.
The standard method
The usual estimate uses the first day of your last menstrual period (LMP) and adds 40 weeks (280 days). This is known as Naegele's rule:
Pregnancy is dated from the LMP because that date is usually known, whereas the exact moment of conception often isn't. Conception typically happens around two weeks *after* the LMP, at ovulation — which is why the actual gestation is about 38 weeks even though the count is 40. The due date calculator applies this and also estimates your conception date and current progress.
Why cycle length matters
The 40-week rule assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts, and so does the due date:
- A longer cycle (say 32 days) means later ovulation, nudging the due date slightly later.
- A shorter cycle pulls it slightly earlier.
That's why the calculator asks for your average cycle length — it adjusts the estimate rather than assuming everyone is exactly 28 days.
How accurate is the date?
It's an estimate, not an appointment. Only around 1 in 20 babies actually arrives on the predicted date; most arrive in the two weeks either side. An early ultrasound (the dating scan) often refines the estimate and can override the LMP date, especially if your cycle is irregular or the LMP is uncertain.
If you like tracking the countdown, pairing the due date with a countdown calculator or working out exact days between dates can be a fun way to follow the weeks. Enter your last period below to see your estimate.
Frequently asked questions
- Why is a due date 40 weeks when pregnancy is 9 months?
- Because it's counted from the first day of your last period, about two weeks before conception. Forty weeks from the last period works out to roughly 38 weeks of actual gestation — a little over nine months — which is where the difference comes from.
- How accurate is a due date?
- It's an estimate. Only about 1 in 20 babies is born on the exact predicted date; most arrive within two weeks either side. An early ultrasound often refines the date and may replace the last-period estimate, particularly with irregular cycles.