How Long Is a Full Pregnancy Term in Months?

By the end of your pregnancy, it may feel like you are going to stay pregnant forever. However, by the end of nine months -- or maybe nine months and some change -- you will finally be holding your little one in your arms. A full-term pregnancy lasts nine months, though most women and doctors prefer to characterize it in weeks for more precise tracking. While some women may go over or under estimates, 40 weeks is still considered a full-term pregnancy.

Defining "Full Term"

A "full-term" pregnancy can actually be anywhere from 37 weeks to 42 weeks. Babies who are born before 37 weeks are considered premature and are at risk of experiencing breathing problems, cardiovascular issues and other serious health conditions. Babies who are born after 42 weeks are prone to the same risks and are also at increased risk of behavioral and emotional problems later in life. While few women actually deliver right on their due date, doctors use 40 weeks as the standard by which to track a woman's progress.

Calculating Due Date

To understand how many weeks have passed in a woman's pregnancy, doctors must date it by learning when the last menstrual period was and then determining a due date. To calculate the due date, doctors add 40 weeks to the first day of the woman's last menstrual period. If the woman has irregular periods or does not remember the first day of her last period, then ultrasound measuring may be used to determine the size of the baby and compare it to averages for other babies for that week of the pregnancy 1.

Gestational Age Versus Fetal Age

Pregnancy is not dated from conception. Because the first day of the last menstrual period is used to calculate a due date, that means a woman is technically considered pregnant two weeks before she has even ovulated. This methodology is used since not all women know exactly when they ovulate or exactly when conception may have occurred. Your baby's gestational age -- the number of weeks since the first day of your last period -- will not be the same as your baby's fetal age -- the number of weeks that have passed since conception.

40 Weeks is Not 10 Months

There is some debate in online forums that a full-term pregnancy is actually 10 months 1. Many people argue this because they say that a month is four weeks and that means that 40 weeks is 10 months. However, under the Gregorian calendar, which is used in the United States, a month is actually about 4.3 weeks. Therefore, 40 weeks is actually nine months -- and perhaps a couple of days, depending on which months are included in the time frame. Women who are a week or two late will be closer to having a 10-month pregnancy 1.