Pregnancy Week Calculator
See the weeks and days of a pregnancy, the trimester and the days left to the due date.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
gestational age = as-of date − first day of the last period first trimester: up to 13 weeks 6 days second trimester: up to 27 weeks 6 days third trimester: from 28 weeks
Weeks are counted from the first day of the last period, not from conception, so the count starts about two weeks before conception.
What to watch out for
Enter the first day of the last period. The as-of date starts on today. For a cycle that is not around 28 days, the due date calculator allows for it. Bleeding in pregnancy needs urgent advice from your maternity unit, midwife or doctor. Heavy bleeding or severe tummy pain is an emergency, so call for emergency help.
Frequently Asked Questions
How do I count pregnancy weeks?
Count from the first day of the last period. From 1 January 2027 to 25 April 2027 is 16 weeks, 2 days, which is in the second trimester, with 166 days to the due date.
When does each trimester start?
The first runs up to 13 weeks and 6 days, the second to 27 weeks and 6 days, and the third from 28 weeks. Definitions vary slightly.
Why does the count start before conception?
Pregnancy dating uses the first day of the last period because that date is usually easier to know than the day of conception.