This is an estimate based on the common Naegele’s rule (280 days from the first day of the last period) and does not account for individual factors. It is not a substitute for care from an obstetrician, who may adjust the date based on an ultrasound or other medical information.

How is the due date calculated?

The most common method (Naegele’s rule) adds 280 days (40 weeks) to the first day of the last menstrual period, assuming a 28-day cycle with ovulation around the middle. If your cycle is longer or shorter, the date is adjusted by the difference.

The three trimesters

Trimester	Weeks
First	Weeks 1 – 13
Second	Weeks 14 – 27
Third	Weeks 28 – 40

Why the actual date may differ

Only a small share of births happen exactly on the estimated date; most occur within two weeks before or after, which is completely normal. An early ultrasound usually gives the most accurate estimate, and your doctor may adjust the date based on it.

Frequently Asked Questions

What if my periods are irregular?

Then this method is less accurate, and an early ultrasound, which measures the baby directly, gives a better estimate of how far along you are.

Can I calculate from the date of ovulation or conception?

Yes. The common alternative is to add 266 days (38 weeks) to the date of ovulation or conception, which gives about the same date if you know it precisely.