Estimate your due date and current week of pregnancy based on the first day of your last menstrual period.
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This calculator uses Naegele's Rule, the standard method for estimating due dates: it adds 280 days (40 weeks) to the first day of your last menstrual period. This assumes a typical 28-day cycle with ovulation around day 14.
For example, a last menstrual period starting January 5, 2026 gives an estimated due date of October 12, 2026, exactly 280 days later.
Only about 5% of babies are born exactly on their estimated due date, most arrive within a two-week window on either side. An ultrasound-based estimate from a healthcare provider is generally more precise than this calculation alone.
Using the date conception is believed to have occurred instead of the first day of the last menstrual period, Naegele's Rule specifically requires the LMP date, not the estimated conception date.
Assuming this estimate is precise for an irregular cycle, Naegele's Rule assumes a typical 28-day cycle, a consistently longer or shorter cycle shifts the actual likely due date.
Treating the due date as a firm deadline rather than the center of a normal range, only a small share of babies arrive exactly on the estimated date.
It's a widely used standard estimate, but actual delivery dates vary. An ultrasound dating scan from a healthcare provider typically offers a more precise estimate, especially for irregular cycles.
If your last period date is uncertain, a healthcare provider can estimate your due date using an ultrasound instead, which is generally more reliable in that situation.
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