Pregnancy Week-by-Week Calculator

Medical convention counts pregnancy from the first day of your last menstrual period (LMP), not from conception. That puts the 'first week' of pregnancy at a point when you weren't actually pregnant yet. Conception happens around week 2; positive home tests show up around week 4. This is why a '40-week' pregnancy is really about 38 weeks of fetal development — the first two weeks are pre-conception by definition.

Gestational age (the standard medical count) is measured from LMP and is what your OB and ultrasound technician will use. Fetal age is from conception and runs about 2 weeks behind. So a 'gestational age 12 weeks' baby has a 'fetal age' of ~10 weeks. Always use gestational age when talking to medical providers; fetal age is more of a textbook abstraction.

First trimester: weeks 1–13. Second trimester: weeks 14–26. Third trimester: weeks 27 to delivery (40+ weeks). The trimester boundaries are reference points more than physiological transitions — there's no sudden change at week 14 or 27. The third trimester is often defined as 'week 28 onward' by some sources; either is fine.

An LMP-based due date is accurate within about ±1 week. A first-trimester ultrasound (8–13 weeks) is the most accurate — typically within ±5 days. After about 20 weeks, ultrasound dating becomes less precise because babies grow at different rates. If your dating ultrasound disagrees with your LMP by more than 7 days in the first trimester, providers usually update the due date to the ultrasound number.

Two common alternatives: (1) Use your provider's due date — they'll set one based on a dating ultrasound at your first prenatal appointment. (2) If you know your conception date (e.g., from ovulation tracking or IVF), use that instead — the calculator handles it. Either is fine; LMP is just the standard input.