Pregnancy Weeks Calculator

How far along a pregnancy is — its gestational age — is measured from the first day of the last normal menstrual period, and is written in completed weeks and days.¹ So “12 weeks 3 days” means twelve full weeks plus three more days have passed since the first day of your last period.

The first day of the last period counts as day 0, so the first seven days of pregnancy are “0 weeks” and a pregnancy enters “1 week” on day 7.¹

Forty weeks is 280 days. By convention, the estimated due date is 280 days — 40 weeks — after the first day of the last period,² and a single-baby pregnancy lasts an average of 40 weeks (280 days) from the first day of the last period to the estimated due date.⁴ In practice, pregnancy normally lasts from 37 to 42 weeks from the first day of the last period.³

Births are grouped by gestational age: early term is 37 weeks 0 days to 38 weeks 6 days, full term is 39 weeks 0 days to 40 weeks 6 days, late term is 41 weeks 0 days to 41 weeks 6 days, and postterm is 42 weeks 0 days and beyond.⁴

Choose “Due date” above and enter it. The calculator works backwards 280 days to the conventional first day of the last period,² then counts how far along you are today. If your due date came from a scan, this keeps your weeks in line with the date your midwife or doctor gave you.

To estimate a due date from your last period, a conception date or an IVF transfer, use the Due Date Calculator.

Ultrasound measurement up to and including 13 weeks 6 days is the most accurate method to establish or confirm how far along a pregnancy is.² If ultrasound dating differs from period-based dating by more than a set number of days, the due date is changed to match the ultrasound.² If your due date has been changed after a scan, enter that due date using the “Due date” option.