A full term pregnancy is 280 days, which equals 40 weeks. Because most months are 30 or 31 days, and not 28 days or four weeks, nine actual calendar months comes closer to the reality of a full term pregnancy.
A full term of pregnancy is 40 weeks. Months usually do not have exactly 4 weeks, so 9 months is more of an estimate. It is more technically correct to go by weeks instead of months, but a lot of people associate pregnancy with months.
The full term is 40 weeks, even though it is not equal to 9 months. Some people go overterm(41-42 weeks) or the baby is born prematurely(under 30 weeks).
