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About Pregnancy
Weeks pregnant, trimester and the estimated due date.
Gestational age is confirmed by crown-rump length in the first trimester, accurate to a few days, while later scans carry wider margins. Prenatal visits in American practice follow a set cadence that changes with each trimester.
American obstetric care counts 40 weeks from the last period, so a pregnancy is already two weeks along at conception, and ACOG schedules screenings against that dating. Only about 4% of babies arrive on the estimated date itself.
American prenatal care dates a pregnancy from the first day of the last menstrual period rather than from conception, which is why a due date lands at 40 weeks and an early ultrasound can shift it by a few days. The fertile window is worked back about 14 days from the next period — an average that suits a 28-day cycle and drifts for anyone whose cycle runs shorter or longer.
Only a small share of births arrive on the estimated date; the figure marks the middle of a normal range of about two weeks either side.
Dating drives every later decision, since screening windows, growth checks and the point at which a pregnancy counts as overdue are all counted from it. A first-trimester scan is the most reliable anchor, and later measurements carry wider margins.
Pregnancy Calculator sits with the rest of the library's health work, and it rests on the same prenatal arithmetic the way a clinical reference table does. Its answer is only as good as the assumptions standing behind it.
The quantities it works from — first day of last period, average cycle length and period length — are the ones a real prenatal question actually turns on, and each carries a convention of its own, set out below.
Health figures in the United States are read against reference tables published by federal agencies and professional bodies, and those tables describe populations rather than individuals. One measurement is a data point; a series taken the same way over months is information.
Clinical guidance also shifts as evidence accumulates, which is why a range that was standard a decade ago may no longer be quoted.