Pregnancy Calculator
See exactly how far along you are — weeks, days, and trimester — with a dated timeline of every pregnancy milestone from missed period to due date.
“How far along am I?” deserves a better answer than a guess. This calculator turns your last period, due date, or conception date into your exact gestational age — weeks, days, and trimester — plus a dated timeline of the milestones ahead: when a test turns positive, when a heartbeat may appear on ultrasound, the anatomy-scan window, viability, term, and your due date.
How pregnancy weeks are counted
Gestational age runs from the first day of your last menstrual period (LMP) — roughly two weeks before conception. It’s a convention built on practicality: most people know their period date far more precisely than their conception date. A full-term pregnancy is 40 weeks (280 days) from the LMP; the due date is simply week 40 on this clock.
Enter whichever anchor you know:
- Last period — the standard clinical count (add your cycle length if it isn’t 28 days).
- Due date — from a scan or your clinician; the calculator works the weeks backward from it, so your timeline matches your official dating.
- Conception date — for IVF or tracked ovulation; conception sits at week 2 of the clock.
The milestone timeline
Every date on the timeline is calculated for your pregnancy: missed period (week 4), possible ultrasound heartbeat (week 6), embryo-to-fetus transition (week 10), first-trimester screening window, second trimester (week 14), the 18–22-week anatomy scan, viability (week 24), third trimester (week 28), early term (37), full term (39), the due date itself (40), and the post-term marker (42) where induction is usually discussed.
The trimester boundaries follow ACOG’s definitions — first trimester through 13w6d, second from 14w0d, third from 28w0d — so this tool always agrees with our due-date calculator.
What the count can and can’t tell you
The weekly clock is precise once the anchor date is right, and a first-trimester ultrasound is the gold standard for that anchor. But milestones are population averages: heartbeats become visible across a range of days, and only about 1 in 20 babies arrives exactly on the due date. Use the timeline to orient — and your clinician’s dating whenever it differs.
Frequently asked questions
How many weeks pregnant am I?
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. Enter your LMP, your due date, or your conception date and the calculator shows your exact gestational age in weeks and days, your trimester, and your progress toward 40 weeks.
Why do the weeks start before conception?
By convention, gestational age starts at the last period — about two weeks before ovulation and conception — because the period date is usually known precisely while conception isn't. That's why a '12 weeks pregnant' pregnancy has a fetus that's been developing for about 10 weeks.
When do the trimesters change?
Using ACOG's definitions: the first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days until birth. The calculator switches trimesters on exactly those boundaries.
How accurate is the week count?
As accurate as the date it starts from. A first-trimester ultrasound is the most reliable dating method — if your clinician has given you an adjusted due date, enter that in the 'due date' mode and the weekly timeline will match your official dating.