"How far along am I?" — what the answer means
Pregnancy is counted in gestational weeks starting from the first day of your last menstrual period — about two weeks before conception. It feels odd, but it's the convention every clinic, scan report and app shares, so a result of "18w 3d" here matches what your provider means by it.
Two ways in
If you know your last period, the calculator counts forward, adjusting for your cycle length (a 35-day cycle pushes everything about a week later). If a clinic already gave you a due date — usually from an early scan, which is more accurate — choose that option and it counts backward instead. The due-date route wins whenever the two disagree.
Trimesters at a glance
The first trimester covers weeks 1–13 — the deep-construction phase when every organ system forms. The second, weeks 14–27, is usually the most comfortable stretch, and it includes the anatomy scan around week 20. The third runs from week 28 to birth, with visits getting more frequent as full term (39 weeks) approaches.
Using the milestone table
The windows below the calculator — dating scan, NT screening, anatomy scan, glucose test, Tdap vaccine, GBS swab — fill in with your personal dates the moment you enter a period or due date. They're the typical windows used in prenatal care; your provider may schedule differently, and that's fine.
A number, not a promise
Everything here is calendar arithmetic on population averages. Babies keep their own schedules: the normal arrival window spans weeks 37 to 42, and only about 4% of babies land on the due date itself.
ListCalc health tools are educational estimates, not medical advice. Dates and ranges here are population averages — your midwife, OB-GYN or doctor confirms what applies to you. See our disclaimer.