“I’m six weeks pregnant” — but pregnant since when, exactly? Gestational age counts from the first day of your last menstrual period (LMP), not from conception, which catches many new parents off guard: it means you’re officially about two weeks “pregnant” before conception even happens. Gestational age drives everything in antenatal care — which tests are due, whether growth is on track, when it’s safe to deliver. Our free gestational age calculator converts your LMP into your current week and day of pregnancy plus your trimester, instantly.
Gestational Age Calculator
How far along are you? Enter the first day of your last period.
How Gestational Age Is Counted
Gestational age (weeks + days) = today − LMP
This convention dates to when LMP was the only reliably known date. The three trimesters:
- First trimester: up to 13 weeks 6 days — organ formation, dating scan, screening tests
- Second trimester: 14 weeks to 27 weeks 6 days — anomaly scan (around 20 weeks), glucose test
- Third trimester: 28 weeks to delivery — growth scans, birth planning
Clinicians write it as “24+3” — twenty-four weeks and three days. Full term is 37–42 weeks by this counting.
How to Use the Gestational Age Calculator
- Enter the first day of your last menstrual period.
- Click Calculate — your gestational age (weeks + days), trimester, and progress appear.
- Bring this to each visit mentally: it helps you understand the scan reports and test timings your doctor orders.
Worked Example
LMP was 1 January 2026 and today is 15 July 2026 — that’s 195 days. 195 ÷ 7 = 27 weeks and 6 days → gestational age 27+6, right at the start of the third trimester borderline (third trimester begins at 28+0).
Notes
- If a first-trimester dating scan gives a different age by more than a week, doctors usually adopt the scan’s date — early embryonic growth is very uniform.
- IVF pregnancies use precise embryo transfer dates instead of LMP — your fertility team gives the adjusted formula.
- Keep a note of your LMP before your first appointment — it’s the first thing the gynaecologist asks.
Frequently Asked Questions
Why does pregnancy count from the last period, not conception?
History and practicality: LMP was knowable, conception wasn’t. Since ovulation usually follows LMP by ~14 days, gestational age runs about two weeks ahead of the baby’s actual (fetal) age.
What’s the difference between gestational age and fetal age?
Gestational age = from LMP (what doctors use). Fetal/conceptional age = from conception, roughly 2 weeks less. Always ask which one a report means.
How accurate is LMP-based gestational age?
Within a few days for regular cycles with a known LMP. Early ultrasound narrows it further — that’s why dating scans are done at 8–13 weeks.
When is my baby “full term”?
Early term 37–38+6, full term 39–40+6, late term 41, post-term 42+. Doctors aim for delivery in the full-term window when possible.
My scan says a different week than my LMP — is something wrong?
Usually not. Small differences (up to a week) reflect cycle variation; bigger ones get reviewed by your doctor, who may revise the due date. Our pregnancy due date calculator shows the matching EDD.
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