The moment a pregnancy is confirmed, one number takes over the calendar: the due date. It shapes appointments, planning, and expectations. Yet it is one of the most misunderstood figures in medicine, because it is an estimate of a range, not a prediction of a single day.
The standard method: Naegele’s rule
The traditional calculation, known as Naegele’s rule, starts from the first day of your last menstrual period (LMP). You add one year, subtract three months, and add seven days — which works out to roughly 280 days (40 weeks) from the LMP. This is why pregnancy is described as “40 weeks” even though conception happens about two weeks after the LMP.
Why start from the last period?
Most people know the date their last period started far more precisely than the date of conception, which can be hard to pin down. Using the LMP gives a consistent, reproducible anchor. The trade-off is that it assumes a regular 28-day cycle with ovulation on day 14 — an assumption that does not hold for everyone.
When ultrasound overrides the date
An early ultrasound measures the size of the embryo or fetus, which in the first trimester is a very reliable indicator of gestational age. If the ultrasound date differs significantly from the LMP date, clinicians usually trust the ultrasound, especially for cycles that are irregular or unknown.
| Milestone | Approx. timing from LMP |
|---|---|
| End of first trimester | ~13 weeks |
| End of second trimester | ~27 weeks |
| Full term begins | 37 weeks |
| Due date | 40 weeks |
The date is a target, not a deadline
Only about 1 in 20 babies is born on the exact due date. A full-term birth is anything from 37 to 42 weeks, and first pregnancies often run a little past the estimate. Thinking of the due date as the center of a several-week window — rather than a fixed appointment — sets far more realistic expectations.
Frequently asked questions
My cycle is irregular — is the LMP date still useful?
It is a starting point, but an early ultrasound will give a more accurate estimate and is the standard way to confirm or adjust the date.
Can the due date change during pregnancy?
Yes. Early ultrasound measurements may revise it. Later scans are less reliable for dating, so the date set in the first trimester usually stands.
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Results are for general information only and are not professional financial, medical, or legal advice.