How it is calculated
- From the first day of your last period (Naegele's rule): add 280 days (40 weeks), adjusted if your cycle is longer or shorter than 28 days.
- From conception: add 266 days (38 weeks).
- IVF: from the transfer date add 263 days for a day-3 embryo or 261 days for a day-5 blastocyst.
Gestational age is always counted from the last period (or its equivalent), so it is about two weeks more than the baby's actual age.
Worked example
If your last period started on 1 March 2026 with a 28-day cycle, the estimated due date is 1 March + 280 days = 6 December 2026.
Frequently asked questions
Do most babies arrive on the due date?
No — only about 4–5% do. Birth between 37 and 42 weeks is considered term.
Which is more accurate, this or an ultrasound?
A first-trimester ultrasound (before 14 weeks) is usually the most accurate, and your doctor may adjust the date based on it.
How many months is a pregnancy?
About 40 weeks, or nine months and a few days. Doctors count in weeks and days, which is what the calculator shows.
When does each trimester end?
First trimester to the end of week 13, second from week 14 to 27, third from week 28 to birth.
My cycles are irregular — what should I do?
Enter your average cycle length, or use your conception date if known, and rely on your early scan date.