What is the expected due date (EDD) calculation based on last menstrual period (LMP)?

The expected due date (EDD) calculation is traditionally based on the last menstrual period (LMP) using the Gestational Age Model, which considers a typical pregnancy lasts about 280 days. This is calculated from the first day of the last menstrual period. Hence, when one says "280 days from the last day of the menstrual period," this aligns closely with standard obstetric practice where a full-term pregnancy is counted as approximately 40 weeks from the first day of the last menstrual period.

In most cases, conception occurs about two weeks after the first day of the last menstrual cycle, thus making the EDD calculation relevant for practitioners to gauge the expected delivery timeframe for the patient accurately. Calculating the EDD this way helps both the healthcare provider and the patient in planning prenatal visits and understanding the pregnancy timeline.