How is a due date calculated?
A due date, or estimated date of delivery, is the day you reach 40 weeks of pregnancy. Doctors count those weeks from the first day of your last period, so the most common way to find it is Naegele's rule: add 280 days to that date. The calculator also adjusts for your cycle length and can work from an ultrasound, a known conception date or an IVF transfer.
Example: a last period that began on January 1, 2027, with a 28-day cycle, gives a due date of October 8, 2027. With a 32-day cycle, ovulation comes 4 days later and the due date moves to October 12.
Formulas
- Last period: first day of the last period + 280 days + (cycle length − 28)
- Conception date: conception + 266 days
- Ultrasound: scan date + 280 days − the length of pregnancy measured at the scan
- IVF: transfer date + 266 days − embryo age (3, 5 or 6 days)
When will the baby really come?
Only a small share of babies are born on their due date. A birth from 37 weeks is term: 37 to 38 weeks is early term, 39 to 40 weeks full term and 41 weeks late term. After 42 weeks a pregnancy is post-term, and most doctors suggest inducing labor before then.
Frequently asked questions
Can my due date change?
Yes. An early ultrasound often refines a date based on the last period, especially if your cycles are irregular.
Are first babies usually late?
Slightly, on average by a few days, but the difference is small and every pregnancy is different.
What if I don't know when my last period started?
Use the date of an ultrasound and the length of pregnancy it showed, or the conception date if you know it.