How Your Pregnancy Due Date Is Calculated (And Why It Is Usually Wrong)
Only 5% of babies arrive on their due date. Here is how the date is determined and what the range really means.
The standard calculation adds 280 days (40 weeks) to the first day of your last menstrual period using Naegele's Rule. This assumes a 28-day cycle with ovulation on day 14 — which describes only about 30% of women. If your cycles are 35 days, your due date is a week later than the standard calculation suggests.
The Reality of Due Dates
Only 5% of babies arrive on their exact due date. About 80% arrive within 2 weeks before or after. First babies tend to arrive 3-5 days late on average. The medical definition of full-term is 39-40 weeks, and healthy babies are commonly born anywhere from 37-42 weeks. Your due date is the center of a window, not a deadline.
When the Due Date Changes
First-trimester ultrasound (8-12 weeks) is the most accurate dating method, within plus or minus 5 days. If the ultrasound date differs from the LMP date by more than 7 days, your provider will typically adjust the due date. After 20 weeks, ultrasound dating becomes less accurate because babies grow at different rates.