Health
Pregnancy Due Dates: What the Calculator Really Estimates
Morecalcs Team 2026-07-22· 6 min
Naegele's rule, why 40 weeks, and how accurate the estimate is.
Naegele's rule
Add 280 days to the first day of the last menstrual period. That's the standard due date.
The assumption
A regular 28-day cycle with ovulation on day 14. Longer or shorter cycles need adjustment — one day of cycle length ≈ one day of due date.
The accuracy
Only 4% of babies arrive on the exact date. About 80% arrive within a two-week window of it.
Ultrasound refines it
A first-trimester dating scan is more accurate than LMP-based calculations and often shifts the date by a few days.
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