How Due Dates Are Calculated
Pregnancy is conventionally measured as 40 weeks (280 days) from the first day of the last menstrual period, even though conception typically occurs about two weeks later. This is Naegele's Rule, the standard method used by most healthcare providers, and it assumes a regular 28-day cycle — the calculator adjusts slightly for longer or shorter cycles.
Why Only About 5% of Babies Arrive on the Exact Due Date
A due date is a statistical estimate, not a prediction — most babies are born within a two-week window on either side of it. Full term is generally defined as 39 to 40 weeks, with "early term" (37-38 weeks 6 days) and "late term" (41 weeks) also considered within a normal range by most medical guidelines.
The Three Trimesters
First trimester (weeks 1-13): Covers conception through early organ development. This is when morning sickness and fatigue are most common, and when the risk of miscarriage is highest.
Second trimester (weeks 14-27): Often called the most comfortable stage — energy typically returns, and fetal movement usually becomes noticeable around weeks 18-20.
Third trimester (weeks 28-40+): The baby gains most of its birth weight during this period, and check-ups typically become more frequent as the due date approaches.
Ultrasound Dating vs. LMP Dating
Early first-trimester ultrasounds (before 14 weeks) are generally considered the most accurate way to date a pregnancy, since fetal growth is highly consistent at that stage. When ultrasound dating and LMP dating disagree by more than about a week, providers typically use the ultrasound date going forward.