Calculating LMP from Due Date: Example for August 9

To estimate Last Menstrual Period (LMP) from a due date:

Formula: Due Date − 280 days (40 weeks) = LMP.

Example: Due date August 9 → LMP ≈ October 18 (previous year).

Calculation: Using a date calculator: 280 days before August 9 is October 18 (non-leap year) or October 17 (leap year prior).

Clinical Note: LMP assumes a 28-day cycle with day 14 ovulation. Ultrasound dating (CRL at 7–13 weeks) is more accurate. If cycles are irregular, LMP is unreliable.