How Pregnancy Due Dates Are Calculated

Your estimated due date (EDD) is calculated using a formula that is almost 200 years old — Naegele's Rule — and it remains the starting point that midwives and obstetricians use today. Understanding how it works helps you know what your date actually represents.

Naegele's Rule

The formula takes the first day of your last menstrual period (LMP), adds one year, subtracts three months, and adds seven days. In simpler terms: your EDD is approximately 40 weeks (280 days) after the first day of your LMP. This assumes a 28-day cycle with ovulation on day 14.

Why your cycle length matters

If your cycle is longer than 28 days, you likely ovulate later, which shifts your actual conception date — and your due date — forward. Cycle-adjusted calculators account for this. Early ultrasound measurements of the fetus (crown-rump length) are the most accurate way to confirm or adjust your EDD.

What "due date" really means

Only about 5% of babies are born on their exact due date. Full term is defined as 39–40 weeks, and normal delivery ranges from 37 to 42 weeks. Your EDD is the midpoint of a window, not a precise appointment. The pregnancy calculator gives you both your EDD and a week-by-week breakdown of your pregnancy timeline.