Pregnancy Due Date Calculator
Estimated due date from your last menstrual period (Naegele).
Input sheet
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An estimated due date anchors the whole timeline of a pregnancy — appointments, milestones, and planning. This tool applies Naegele's rule, the standard method based on the first day of your last menstrual period.
How it works
Naegele's rule: due date is the first day of the last period plus 280 days (40 weeks).
Naegele's rule adds 280 days (40 weeks) to the first day of the last period. The calculator also estimates current gestational age in weeks and the approximate conception date, about two weeks after the period began.
The method assumes a regular 28-day cycle with ovulation around day 14. If your cycles are longer, shorter, or irregular, the true date can shift, which is why an early ultrasound often refines the estimate.
Estimated due date = first day of last period + 280 days. Approximate conception = last period + 14 days.
Worked examples
A last period beginning January 1, 2026. → Estimated due date October 8, 2026.
January 1 + 280 days = October 8; conception ≈ January 15.
A last period beginning March 10. → Estimated due date about December 15.
March 10 + 280 days lands in mid-December.
Tips & gotchas
- Note the date confidently only if your last period was typical; spotting or an unusual cycle can throw the estimate off.
- An early dating ultrasound (around 8-13 weeks) is usually more accurate than the LMP method and may revise the date.
- Use the due date as a center point — a normal full-term birth can happen anytime in a multi-week window around it.
- Track gestational age in weeks rather than the single due date, since most prenatal care is organized by week.
FAQ
How likely is my baby to arrive on the exact due date?
Only about 1 in 20 babies arrive on the predicted day. Most full-term births occur within roughly two weeks on either side, so treat the date as a midpoint, not a deadline.
What if my cycle isn't 28 days?
Naegele's rule assumes a 28-day cycle. Longer or shorter cycles shift ovulation and the true due date, which is why early ultrasound dating is more reliable for irregular cycles.
Why use the last period instead of the conception date?
Most people know when their last period started but not the exact day of conception. The LMP method gives a consistent, easy-to-pinpoint reference that clinicians use worldwide.
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