Due date formula
Due date = LMP + 280 days + (cycle length − 28)
Due date = Conception date + 266 days
Due date = Conception date + 266 days
Example: If your last period started on January 1, 2026 and your cycle is 28 days, your estimated due date is October 8, 2026. With a 32-day cycle, ovulation usually happens about 4 days later, so the due date moves to October 12, 2026.
Pregnancy trimesters
| Trimester | Weeks | Milestones |
|---|---|---|
| First | Weeks 1–13 | Organs form; first prenatal visit and dating ultrasound |
| Second | Weeks 14–27 | Anatomy scan around 18–22 weeks; movement felt |
| Third | Weeks 28–40+ | Rapid growth; full term from 39 weeks |
A pregnancy is considered full term from 39 weeks 0 days to 40 weeks 6 days, early term from 37 weeks, and preterm before 37 weeks.
This tool gives an estimate only. Your healthcare provider will confirm your due date, usually with an ultrasound.
Frequently asked questions
How is a due date calculated?
Most providers use Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). If your cycle is longer or shorter than 28 days, the date shifts by the difference. From a known conception date, add 266 days (38 weeks).
How accurate is my due date?
Only about 4–5% of babies arrive on their exact due date. Most are born within about two weeks either side. An early ultrasound (before 14 weeks) is the most accurate way to date a pregnancy and may adjust your due date.
Why is pregnancy counted from my last period?
Because the date of the last period is usually known, while the exact day of ovulation and conception usually isn’t. That means you are considered “2 weeks pregnant” around the time of conception.
What if I conceived through IVF?
For IVF, the due date is based on the embryo transfer date: add 266 days minus the embryo’s age at transfer (for example, 261 days after a day-5 blastocyst transfer). Your clinic will confirm the exact date.