"How far along am I?" has a precise answer: pregnancy is dated from the first day of your last menstrual period (LMP), and each completed 7 days is a week. This calculator counts it either from your LMP or backwards from a due date you already have.
Why week 1 starts before conception
Medical dating uses LMP because it's a date most people actually know — conception, roughly two weeks later, usually isn't observable. The consequence: "40 weeks pregnant" contains about 38 weeks of fetal development, and you're already called "4 weeks pregnant" around the time a period is first missed.
Trimesters and the timeline
The trimester boundaries used by providers: first = weeks 1–13, second = 14–27, third = 28 to birth. The progress bar marks both transitions, and the milestone line always shows the next commonly referenced marker — heartbeat on ultrasound near week 6, anatomy scan at 18–22, the viability reference near 24, early term at 37, full term at 39.
How the two modes relate
Due date is just LMP + 280 days (Naegele's rule), so the two modes are inverses: given one, the calculator derives the other. If you only know an approximate positive-test date, that's typically around week 4–5 — count back to estimate your LMP. For the due-date-focused version of this math with a full pregnancy timeline, see the due date calculator.
The honest error bars
LMP dating assumes a 28-day cycle with ovulation on day 14. Longer, shorter or irregular cycles shift the true week — which is why first-trimester ultrasounds commonly re-date pregnancies by several days, and why your provider's dating wins over any calculator.
Educational only — not medical advice; confirm dating with your healthcare provider.