About the due date calculation
Due dates are estimated from the first day of your last menstrual period, because that date is far easier to pin down than conception. Only about 4% of babies actually arrive on the estimated date — treat it as the centre of a window, not a deadline.
Formula
Naegele's rule: due date = first day of last period + 280 days
Adjusted: + 280 + (cycle length − 28) days
Pregnancy is dated from the LMP, roughly two weeks before conception.Worked example
A last period starting 1 January with a 28-day cycle gives a due date around 8 October — 280 days later.
Frequently asked questions
How accurate is a due date from the last period?
It is an estimate with a wide spread. Around 4% of births happen on the exact date, and roughly 80% fall within the two weeks either side. A first-trimester dating ultrasound is more accurate and will usually take precedence.
Why adjust for cycle length?
Naegele's rule assumes a 28-day cycle with ovulation on day 14. If your cycles run longer, you ovulate later, so conception happened later and the due date shifts accordingly.
Why is pregnancy dated from before conception?
Because the LMP is an observable date and conception usually is not. It means the first two weeks of a "40-week" pregnancy happen before you are actually pregnant — real gestation is about 38 weeks from conception.
Sources
- ACOG Committee Opinion 700 — Methods for estimating the due date (2017)