Due Date Calculator
Estimate your baby's due date from your last period or conception date.
Quick answer
A pregnancy due date is estimated as 280 days (40 weeks) from the first day of the last menstrual period, or 38 weeks from conception (Naegele's rule). Only about 1 in 20 babies arrive exactly on the due date — it's a central estimate, not a deadline.
Naegele's rule
The classic estimate adds 280 days (40 weeks) to the last period's first day, assuming a 28-day cycle with ovulation at day 14. Counting from conception uses 266 days; IVF transfers are the most precise, dated from the embryo's exact age.
A window, not an appointment
Term spans 37–42 weeks: about 60% of babies arrive within a week of the due date, and first pregnancies tend to run a few days late. Treat the date as the center of a month-wide window.
Frequently asked questions
- How does the Due Date Calculator work?
- A pregnancy due date is estimated as 280 days (40 weeks) from the first day of the last menstrual period, or 38 weeks from conception (Naegele's rule). Only about 1 in 20 babies arrive exactly on the due date — it's a central estimate, not a deadline.
- Is the Due Date Calculator free to use?
- Yes. The Due Date Calculator is completely free, runs entirely in your browser, and requires no account or sign-up. Your inputs never leave your device.
Sources & methodology
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