Pregnancy Week Calculator

"How many weeks pregnant am I?" has a slightly surprising answer: pregnancy is counted from the first day of your last menstrual period, not from conception, which happens about two weeks later. That convention is used everywhere in maternity care, so your "weeks" here will match what your provider uses.

Your gestational age places you in a trimester — first (up to 13 weeks 6 days), second (14 to 27 weeks 6 days), or third (28 weeks onward) — and tells you roughly how far you are from your estimated due date, which is 40 weeks from your last period.

Because it's based on your last period, this is an estimate. If a first-trimester ultrasound gave you a different date, use that: it dates a pregnancy more accurately, and your provider's dating is what all your care is scheduled around.

Method. Your gestational age is today's date minus the first day of your last menstrual period, expressed in weeks and days. Your estimated due date is that date plus 280 days (40 weeks), and your trimester follows the standard boundaries.

Source. Gestational-age counting from the last period and the trimester definitions are the standard used by the American College of Obstetricians and Gynecologists.

Confidence. We label this estimate-only: last-period dating is an approximation, and a first-trimester ultrasound is more accurate. Always defer to your provider's dating.