How to Calculate Your Due Date
Your due date is estimated by adding 40 weeks (280 days) to the first day of your last menstrual period. This assumes a standard 28-day cycle. While only 5% of babies arrive on their actual due date, having an estimated date helps you and your doctor monitor the baby's growth and schedule necessary prenatal tests.
Why is the Due Date Calculated from the Last Period?
Most women don't know the exact date they conceived, but they do remember the first day of their last period. Doctors use this date as the starting point for your 40-week pregnancy timeline, even though you aren't technically pregnant for the first two weeks of that period.