How Due Dates Are Actually Calculated

Naegele's rule, the 40-week count, and why your due date is really a due month. A clear look at the maths behind that magic date.

That single date on your fridge feels so precise — but where does it come from, and how seriously should you take it? Here’s what’s going on behind the scenes.

The 40-week count

Pregnancy is counted as 40 weeks (280 days) from the first day of your last menstrual period (LMP). This is known as Naegele’s rule. It sounds odd, because you weren’t actually pregnant during the first two weeks — conception usually happens around two weeks after your period starts. But because most people know when their last period began (and not the exact moment of conception), the LMP makes a convenient, consistent starting line.

Why cycle length matters

The 280-day rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, ovulation shifts — and so does your due date. That’s why our due date calculator lets you set your average cycle length.

Conception and IVF dating

If you know your conception or ovulation date, we count 266 days forward instead. For IVF, the conception date is known almost exactly, so we adjust for the embryo’s age at transfer (day-3 or day-5) — which makes IVF due dates among the most accurate.

A due month, really

Here’s the part worth tattooing on your heart: only about 1 in 20 babies arrive on their exact due date. Most are born within two weeks either side. Think of it as a due month, not a deadline. Your provider’s dating — especially from an early ultrasound — is the most reliable, and always takes priority.

Ready to find yours? Calculate your due date →