The unborn baby spends around 38 weeks in the uterus, but the average length of pregnancy, or gestation, is counted at 40 weeks. Pregnancy is counted from the first day of the woman's last period, not the date of conception which generally occurs two weeks later.

Why do they say pregnancy is 9 months when it's 40 weeks?

That's because months vary in days. Averaging 365 days over 12 months, your average month lasts 30 days and 10 hours. So 280 days (or 40 weeks) is actually nine months plus nearly a week.

Why is pregnancy 40 weeks or 36?

There are 40 weeks (or 280 days) in a pregnancy. Your due date is calculated starting from the first day of your last menstrual period (even though most pregnancies are not conceived until two weeks later). Keep in mind that only about 30 percent of pregnancies will reach precisely week 40.