Naegele's Rule
Due Date = LMP + 7 days − 3 months + 1 year
Equivalent to: Due Date = LMP + 280 days
Common Mistakes
- Using the conception date instead of LMP — most calculators and doctors default to LMP-based dating unless conception date is precisely known (e.g., via IVF).
- Assuming the due date is exact — only about 5% of babies arrive on the calculated date; a range of 38-42 weeks is entirely normal.
- Not accounting for irregular cycles — Naegele's Rule assumes a standard 28-day cycle; longer or shorter cycles shift the actual due date.
Quick Reference
Trimester breakdown: Weeks 1–13 (First), Weeks 14–27 (Second), Weeks 28–40 (Third).
Frequently Asked Questions
Why 280 days specifically?
This represents 40 weeks, the historically observed average length of human pregnancy counted from the last menstrual period, even though actual conception occurs roughly two weeks after LMP.
How common is it to deliver exactly on the due date?
Quite rare — only about 5% of babies arrive on their exact calculated due date, with the large majority born within a week or two before or after.
