How is a pregnancy due date calculated?

The 280-day rule

The convention is known as Naegele's rule, after the nineteenth-century obstetrician who popularised it. You take the first day of the last menstrual period and add 280 days, which is 40 weeks.

The traditional shorthand is to subtract three months and add seven days and a year, which arrives at almost the same answer. Adding 280 days directly is exact and is what this site does.

The rule became standard because it needs only one piece of information that most people actually have: when their last period started. Conception itself is rarely observed.

Why the count starts before conception

This is the part that surprises people. Gestational age is counted from the last menstrual period, and in an average cycle conception happens around two weeks later. A pregnancy therefore lasts about 38 weeks from conception but is described as 40 weeks.

One practical consequence: by the time a pregnancy test turns positive, usually around a missed period, you are typically already described as four weeks pregnant. Nothing has gone wrong with the arithmetic.

Adjusting for cycle length

The 280-day figure assumes a 28-day cycle with ovulation on day 14. Cycle length varies mostly in the follicular phase, the stretch before ovulation, while the luteal phase after ovulation stays close to 14 days for most people.

So a 35-day cycle means ovulation around day 21, a week later than the rule assumes, and the due date moves a week later with it. A 24-day cycle moves it four days earlier. Calculators that ignore cycle length can therefore be a week out for people whose cycles are long.

Dating from conception, IVF or ultrasound

If the conception date is known, adding 266 days skips the cycle assumption altogether. IVF is more precise again, because the embryo's age on the day of transfer is documented: a day 5 blastocyst transfer means adding 261 days.

The most accurate method of all is an ultrasound in the first trimester, which measures the pregnancy directly rather than inferring it from a date. Where a scan and a calculation disagree, the scan is usually the one care teams keep.

• Last period: add 280 days, adjusted for cycle length
• Conception or ovulation date: add 266 days
• Day 5 embryo transfer: add 261 days
• First-trimester ultrasound: measured, and generally the most accurate

What the date does not mean

Fewer than one in twenty babies are born on their estimated due date. Birth anywhere between 37 and 42 weeks is common, and the due date sits in the middle of that spread rather than at its end.

Treating it as a range rather than a deadline is both more accurate and considerably less stressful.