How the due date is calculated
- Choose whether you know your last period or conception date.
- Enter the date and, for the last period, your usual cycle length.
- See your due date, current week and trimester.
Naegele's rule
The due date is estimated as 280 days after the first day of the last period, adjusted for cycles longer or shorter than 28 days, or 266 days after conception.
Weeks of pregnancy
Pregnancy is counted from the last period, about two weeks before conception, so at conception you are already considered two weeks pregnant.
Ultrasound dating
An early ultrasound measures the baby and can move the due date by several days. Your care provider will tell you which date to use.
Frequently asked questions
How is the due date calculated?
Naegele's rule adds 280 days (40 weeks) to the first day of your last period, adjusted if your cycle is longer or shorter than 28 days. From conception, it adds 266 days.
Why is pregnancy counted from my last period?
Ovulation dates are rarely known, so pregnancy is dated from the last period, about two weeks before conception.
How accurate is a due date?
It is an estimate. A first-trimester ultrasound is the most accurate way to date a pregnancy.
When does each trimester start?
The first trimester is weeks 1โ13, the second weeks 14โ27 and the third from week 28 until birth.