Formula
Due date = first day of last period + 280 days (40 weeks)Cycle adjustment: + (cycle length − 28) daysFrom conception: due date = conception + 266 days (38 weeks) How it works
Pregnancy is conventionally dated from the first day of the last menstrual period (LMP), about two weeks before conception actually happens. A full-term pregnancy is counted as 40 weeks, or 280 days, from that date. This rule is known as Naegele’s rule.
The standard rule assumes a 28-day cycle with ovulation on day 14. If your cycles are usually longer or shorter, ovulation shifts by the same amount, so the calculator moves the due date to match. If you know the date you conceived — for example after fertility treatment — the due date is 266 days (38 weeks) after conception.
Weeks 1–13 are the first trimester, weeks 14–27 the second and week 28 onward the third. A first-trimester ultrasound measurement is usually the most accurate way to date a pregnancy, so your provider’s date takes precedence over this estimate. This tool is for information only and is not medical advice.
Frequently asked questions
Why does pregnancy count from before conception?
Most people know when their last period started but not exactly when they ovulated. Counting from the period gives everyone a consistent reference point, so "week 6" means the same thing to every clinician.
How accurate is a due date?
It is an estimate. Around 4% of babies are born on the exact day; roughly 80–90% arrive between 37 and 42 weeks.
What if my cycles are irregular?
Date-based estimates are less reliable with irregular cycles. An early ultrasound gives a better estimate.