How Pregnancy Is Dated
Pregnancy is measured in weeks starting from the first day of your last menstrual period, or LMP, even though conception happens about two weeks later. This is why a pregnancy is typically described as lasting 40 weeks, or about 280 days. A pregnancy calculator uses your LMP date to estimate your due date and your current week of pregnancy.
The due date uses Naegele's rule: add 280 days to the first day of your last period. Because very few babies arrive exactly on their due date, this is always an estimate. Most babies are born within two weeks before or after the predicted date.
Why a Due Date Is Only an Estimate
Conception timing, cycle length, and individual growth all affect when a baby is born. An early ultrasound can refine the due date, especially if your cycles are irregular. Doctors use the due date as a reference point for scheduling checkups and monitoring development rather than as a guarantee.
Understanding that the date is approximate can reduce anxiety as the due date approaches. Many first-time parents are surprised to learn that a typical pregnancy naturally spans a range of roughly 37 to 42 weeks.
The Three Trimesters
Pregnancy is divided into three trimesters. The first trimester runs from week 0 to week 13 and includes the earliest stages of development. The second trimester runs from week 14 to week 27 and is often when movement is first felt. The third trimester runs from week 28 until birth and involves the final growth and preparation for delivery.
Key milestones help you track progress. The first-trimester screening often happens around week 12, while the detailed anatomy scan is usually scheduled around week 20. For more timeline tools, try our due date calculator or our period calculator.