How do you work out the length of a pregnancy in weeks?

You might have heard people talking about what to expect in different weeks of a pregnancy. Or use weeks as a way of explaining how far along a pregnancy is. This is sometimes known as the gestational age of a pregnancy.

This is worked out from the first day of your last period. For example, if you are 4 weeks pregnant, it means your last period started 4 weeks ago. If your menstrual cycle lasts 28 days, you would usually ovulate around 2 weeks after the first day of your last period. You’ll conceive on the days around your ovulation date.

If you do not know exactly when you got pregnant, your doctor or clinician will estimate how many weeks pregnant you are. They do this by looking at the times of your menstrual cycle and examining you. They might do an ultrasound scan as well.

If you , know when you’re ovulating or do not have sex regularly, you might already know exactly when you got pregnant. If you do, tell your doctor or clinician.

Reviewed by Helen Burkitt. Senior Sexual Health and Contraception Nurse
Last updated at: 02 February 2024
Published on: 02 May 2023