Pregnancy week calculator
Find how many weeks and days pregnant you are from your last period or due date.
Direct answer: pregnancy weeks are counted from the first day of your last menstrual period. A due date is conventionally 280 days later, so today's date minus that starting date gives an estimated week and day.
We calculate in your browser. Your date is not uploaded or saved.
Choose the date you know to see your week and trimester.
How pregnancy weeks are counted
Gestational age begins on the first day of the last menstrual period—not the day of conception. That shared convention gives families and clinicians one consistent timeline. It means that at conception, a pregnancy is already dated at roughly two weeks.
ACOG's clinical guidance on pregnancy dating states that the estimated due date is conventionally 280 days from the LMP and that first-trimester ultrasound is the most accurate way to establish or confirm gestational age.
Common questions
How many weeks pregnant am I from my last period?
Count from the first day of your last menstrual period to today. Pregnancy is conventionally dated across 40 weeks, even though conception usually happens about two weeks after that starting date.
Which trimester am I in?
This calculator labels weeks 0–13 as the first trimester, 14–27 as the second, and week 28 onward as the third. Your care team may describe the boundaries slightly differently.
Is gestational age from an LMP exact?
No. Cycle variation, uncertain dates and ovulation timing can change the estimate. ACOG says first-trimester ultrasound is the most accurate method to establish or confirm gestational age.
Want the date first? Try the due date calculator, or browse every free pregnancy and baby tool.