Pregnancy Due Date Calculation: Why Your Doctor Measures From Before Conception

The standard pregnancy due date calculation starts from the first day of your last menstrual period, not from conception. This confuses nearly everyone because it means you are "two weeks pregnant" before the egg is even fertilized. The reason is historical, practical, and still medically sound. Naegele's Rule The standard formula, developed by Franz Naegele in the early 1800s, is: Due date = Last Menstrual Period (LMP) + 1 year - 3 months + 7 days If your LMP was March 1, 2025: March 1, 2025 + 1 year = March 1, 2026 March 1, 2026 - 3 months = December 1, 2025 December 1, 2025 + 7 days = December 8, 2025 This assumes a 28-day menstrual cycle with ovulation on day 14. From the LMP, that puts conception at approximately week 2, meaning the first two "weeks of pregnancy" predate conception. The reason LMP is used instead of conception date is practical: most people can identify the start of their last period with reasonable certainty. The exact date of conception is usually unknown becaus