How is a due date calculated?
Doctors usually count pregnancy from the first day of your last menstrual period (LMP), about two weeks before conception. The standard due date is 280 days (40 weeks) after the LMP — this is called Naegele’s rule. If your cycles are longer or shorter than 28 days, the calculator adjusts the date.
| Method | Due date = |
|---|---|
| Last period (28-day cycle) | LMP + 280 days |
| Conception date | Conception + 266 days |
| IVF day-5 transfer | Transfer date + 261 days |
| IVF day-3 transfer | Transfer date + 263 days |
| Ultrasound | Ultrasound date + (280 days − gestational age at scan) |
How accurate is a due date?
Only about 4–5% of babies arrive on their exact due date. Most are born within two weeks either side. An early ultrasound (in the first trimester) is the most accurate way to date a pregnancy, and your provider may update your due date after it. IVF dates are very precise because the conception date is known.
Trimesters
| Trimester | Weeks |
|---|---|
| First | Week 1 to 13 weeks 6 days |
| Second | Week 14 to 27 weeks 6 days |
| Third | Week 28 to birth |
A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days, early term at 37–38 weeks and late term at 41 weeks (per ACOG definitions).
Trying to conceive? Find your fertile days with the ovulation calculator.
Frequently asked questions
How is my due date calculated?
By adding 280 days (40 weeks) to the first day of your last period, adjusted for your cycle length.
How many weeks pregnant am I?
Enter your last period date — the calculator shows your current week and day.
Can my due date change?
Yes. Your provider may adjust it after an early ultrasound.
How common is it to give birth on the due date?
Only about 1 in 20 babies arrive exactly on their due date.