How the date is worked out
Naegele’s rule: 280 days from the first day of the last menstrual period. That counts pregnancy from roughly two weeks before conception, which is why "how many weeks pregnant" is always about two weeks more than time since conception — a discrepancy that confuses almost everyone at first.
It assumes a 28-day cycle with ovulation on day 14. Longer or shorter cycles shift the date accordingly.
Only about 4% arrive on the date
The due date is the midpoint of a distribution, not an appointment. Roughly 4% of babies are born on it; the large majority arrive within two weeks either side. A "due week" is a more honest way to think about it than a due day.
A dating ultrasound in the first trimester is more accurate than any calculation from a period date, and where the two disagree the scan is what maternity care uses.
What this page is not
An estimate for planning, and nothing more. Antenatal care, any concern about the pregnancy, and decisions about anything at all belong with a midwife or doctor. If your cycle is irregular or you are unsure of dates, a scan is the answer rather than a better formula.
Frequently asked questions
How likely is the baby to arrive on the due date?
About 4%. It is the midpoint of a range, and most births fall within two weeks either side.
Why am I "two weeks pregnant" before conception?
Counting starts from the first day of your last period, which is roughly two weeks before ovulation. It is a convention, and it catches everyone out.
My cycle is not 28 days. Is this still right?
It will be off. The rule assumes a 28-day cycle with ovulation on day 14. A dating scan is more reliable for irregular cycles.
Which is more accurate, this or a scan?
A first-trimester scan, clearly. Where the two disagree, maternity care uses the scan.
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