Step-by-Step Calculation (Naegele's Rule)
- Take the first day of your last menstrual period.
- Add 7 days.
- Subtract 3 months.
- Add 1 year.
Worked Example
Last menstrual period started March 10.
Add 7 days: March 17
Subtract 3 months: December 17
Add 1 year: December 17 (next year) — your estimated due date, 40 weeks from the LMP.
This mathematically shortcuts the same 280-day calculation without needing to count individual days.
Frequently Asked Questions
What if my cycle isn't exactly 28 days?
Naegele's Rule assumes a standard 28-day cycle — if yours is longer or shorter, your actual due date may shift by a few days in either direction from the calculated estimate.
Is an ultrasound more accurate than this calculation?
Early first-trimester ultrasounds are generally considered more accurate for dating a pregnancy than LMP-based calculation alone, especially for women with irregular cycles.
