Pregnancy due date calculator
Due date from last period or conception date.
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How it's calculated
Doctors date a pregnancy from the first day of the last period, not from conception, and estimate the due date 280 days (40 weeks) later: Naegele’s rule. If your cycle is longer or shorter than 28 days, ovulation shifts by the same amount, so the due date moves a day per day of difference. Weeks pregnant is the days since that start divided by seven; the trimesters change at 13 and 27 weeks. About 4% of babies arrive on the due date itself; most come within two weeks either side.
How accurate is a due date from the last period?
Within about a week for regular cycles. An early ultrasound (before 14 weeks) is more precise and your provider will use it if the two disagree by more than a few days.
What if I don't know the date of my last period?
Enter your best guess and adjust at the first scan. If you know the conception date instead, use the date 14 days earlier as the start of the last period.
Why does the tool count from the period and not conception?
Because that is how medical dating works: "8 weeks pregnant" means 8 weeks since the last period, about 6 weeks since conception. Every milestone chart and test window uses the same count.
Results are estimates for informational purposes only and are not financial, medical or legal advice.