A pregnancy is considered full term at 39 weeks. This means it’s best not to schedule your baby’s delivery before that point unless there’s a medical reason to do so.

On average, pregnancies last about 40 weeks (280 days) from the first day of a woman’s last menstrual period. That’s how your practitioner will most likely estimate your due date.

Caregivers previously believed that babies born between 37 and 42 weeks were equally likely to be born healthy. For that reason, experts considered pregnancies to be full term at 37 weeks.

Full term is now 39 weeks because research has shown that babies born at 39 weeks were less likely to have certain health problems than those born at 37 weeks. (Babies born at 40 weeks were not significantly healthier than those born at 39 weeks.)