How to use the Pregnancy Due Date Calculator
- Choose your calculation method — usually the first day of your last period.
- Enter the date and, for the period method, your average cycle length.
- See your estimated due date, current week and trimester milestones.
- Optional: download estimated week markers as calendar events.
How is the due date calculated?
Doctors use Naegele’s rule: add 280 days (40 weeks) to the first day of your last menstrual period. If your cycle is longer or shorter than 28 days, the date is shifted by the difference. From a known conception date, add 266 days. For IVF, count from the transfer date and embryo age.
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 (before 14 weeks) is the most accurate way to date a pregnancy, and your provider may adjust your due date after it.
Trimesters
- First trimester: weeks 1–13
- Second trimester: weeks 14–27
- Third trimester: week 28 to birth
This calculator gives general estimates for healthy adults and is not medical advice. Talk to a doctor or registered dietitian for personal guidance.
Frequently asked questions
Why is pregnancy counted from my last period?
Because the date of ovulation is usually unknown, while the start of your period is easy to remember. This adds about two weeks before conception.
What is “full term”?
Full term is 39 weeks 0 days to 40 weeks 6 days.