Due Date Calculator

This calculator provides an estimate and is not medical advice. Consult your healthcare provider for an accurate due date.

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Why Pregnancy Is Dated From Before Conception

A pregnancy's "due date" is conventionally counted from the first day of the last menstrual period (LMP), not from conception itself — conception typically happens about two weeks after LMP, near ovulation. That convention exists because LMP is a date most people can identify with reasonable confidence, while the exact moment of conception usually isn't. Naegele's Rule, the method behind this calculator, has been the standard estimation method since the early 1800s.

The Formula

Estimated Due Date = LMP + 280 days (40 weeks)

280 days reflects an average 40-week gestation counted from LMP, based on a standard 28-day menstrual cycle. It's an estimate, not a guarantee — only a minority of babies are born on their exact calculated due date, with a full-term range spanning roughly 37 to 42 weeks.

A Worked Example

For an LMP of January 1, 2026, adding 280 days lands on October 8, 2026 — the estimated due date under Naegele's Rule.

Gestational Age Milestones

Common pregnancy trimester and term boundaries counted from LMP
MilestoneWeeks from LMP
First trimesterWeeks 1–13
Second trimesterWeeks 14–27
Third trimesterWeeks 28–40
Early termWeeks 37–38
Full termWeeks 39–40
Late termWeek 41
Post-termWeek 42+
Note: This is an estimate for informational purposes only and is not medical advice. An ultrasound-based dating scan in early pregnancy is generally more precise than LMP-based calculation alone, especially for irregular cycles. Consult your healthcare provider for an accurate due date.

Where This Gets Used

  • Prenatal appointment scheduling — screening tests, ultrasounds, and other prenatal care are timed around gestational age.
  • Planning parental leave — a starting estimate for coordinating time off work around the expected delivery window.
  • Tracking pregnancy progress — converting today's date into "weeks and days pregnant" for app trackers, journals, or conversations with a provider.

How to Use This Calculator

  1. Enter the first day of your last menstrual period.
  2. Select Calculate to see the estimated due date and current gestational age in weeks and days.

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