How due dates are calculated

The anchor: 280 days from the last period

Pregnancy is counted from the first day of the last menstrual period, about two weeks before conception in a typical cycle. The estimated due date is 280 days, or 40 weeks, after that day.^1,2 Every other method is converted to this same scale, which is why an IVF pregnancy can be “6 weeks” four weeks after transfer.

Naegele’s rule, and why we count days instead

The traditional shortcut, Naegele’s rule, adds a year, subtracts three months and adds seven days. Because months have different lengths, it can land a day or so away from a true 280-day count.³ Our calculator always counts actual days.

Naegele’s rule would give March 8, 2028. The 280-day count gives March 7, because February 2028 has 29 days.

Cycle-length adjustment

If your cycle is regularly longer or shorter than 28 days, the usual adjustment shifts the due date by the difference, on the assumption that ovulation shifted by the same amount.^3,4

Conception

Conception sits at 2 weeks 0 days on the pregnancy scale, so the due date is 266 days later.¹

IVF

With IVF, the egg retrieval date is the fertilization date. An embryo transferred on day 3 or day 5 is already that old, so the due date is 263 days after a day-3 transfer and 261 days after a day-5 transfer.¹

Ultrasound

A scan gives gestational age on the scan date. Add 280 days to the scan date and subtract the age. A crown-rump length measured up to 13 weeks 6 days is the most accurate way to date a pregnancy.¹ If the report gives only a crown-rump length, the Robinson equation converts it: days = 8.052 × √CRL(mm) + 23.73.^5,6

Which date wins

Clinicians set one due date as early as possible and then keep it. For IVF pregnancies they use the IVF date. Otherwise they use the period date unless an early ultrasound differs by more than set thresholds, in which case they use the ultrasound date.¹ See the thresholds in ultrasound vs last period.

Calendar dates, not clocks

We do all of this as whole calendar days. Software that adds 280 × 24 hours to a timestamp can slip a day across a daylight-saving change or time zone. Our methodology describes how we test for that.