How it works
Estimated due date from the last menstrual period (LMP) follows Naegele's rule, named for Franz Karl Naegele (19th century): add one year, subtract three months, add seven days to the LMP — equivalent to adding exactly 280 days (40 weeks) from LMP. The assumption is a 28-day cycle with ovulation on day 14, which is a population average, not every person's biology. Only about 4–5% of babies arrive on the exact due date; most deliver between 38 and 41 weeks. The engine validates the LMP date, adds 280 days, and reports the due date as YYYY-MM-DD. It also computes gestational age from LMP to a fixed reference date (September 15, 2026 in the shipped defaults) as weeks pregnant and counts days remaining until the due date. Worked example: LMP December 20, 2025. Due date = December 20 + 280 days = September 26, 2026. From December 20, 2025 to September 15, 2026 is 269 days ≈ 38 weeks 3 days pregnant, with 11 days remaining until the due date. First-trimester ultrasound dating (crown-rump length) can shift the due date by several days and often supersedes LMP when the difference exceeds a week — this calculator does not replace clinical dating. Irregular cycles, hormonal contraception recently stopped, or uncertain LMP dates reduce accuracy; early ultrasound is the gold standard when available.
Formula
Due date = LMP + 280 days (40 weeks) | Weeks pregnant = days since LMP ÷ 7
Tips
- Use the first day of bleeding as LMP, not spotting or the last day of the period.
- Early ultrasound dating overrides LMP when they disagree by more than 5–7 days.
- Full term is 37–42 weeks — the due date is the midpoint, not a deadline.
- Irregular cycles longer than 28 days often push true ovulation later than Naegele assumes.
Frequently asked questions
- How is a due date calculated from LMP?
- Naegele's rule: LMP + 280 days (40 weeks). LMP December 20, 2025 → due date September 26, 2026.
- How many weeks pregnant am I on the defaults?
- With LMP December 20, 2025 and reference date September 15, 2026, gestational age is approximately 38 weeks and 3 days — about 11 days before the due date.
- Is the due date exact?
- No — it is an estimate. Normal delivery spans 37–42 weeks. Only a small fraction deliver on the calculated day.
- What if my cycles are not 28 days?
- Naegele assumes a 28-day cycle. Longer cycles usually mean later ovulation, so an ultrasound in the first trimester gives a more reliable due date than LMP alone.