How the estimated due date is calculated
The most widely used method is Naegele's Rule: 280 days—exactly 40 weeks—are added to the first day of your last menstrual period. If your menstrual cycle is usually longer or shorter than 28 days, the date is adjusted proportionally, as different cycle lengths shift the probable ovulation day.
Why the count starts before conception
This often confuses people: gestational age is calculated from the first day of your last period, not from the day conception actually occurred. Because ovulation usually happens about two weeks after the start of a period, the first two counted weeks of "pregnancy" actually precede conception. It is a medical convention, not a calculation error.
Trimesters of pregnancy
The first trimester spans from week 1 to 13, the second from week 14 to 27, and the third from week 28 until delivery, typically around week 40. Each trimester comes with different developmental milestones and prenatal care guidelines.
The due date is an estimate
Fewer than 5% of babies are born on their exact calculated due date. Most are born within a window of about two weeks before or after. A first-trimester ultrasound, which directly measures embryo size, usually refines this estimate with higher accuracy than calculations based solely on your last period.