A baby born before 37 weeks of pregnancy is considered premature or born too early. The number of premature births in the U.S. is rising. Twins and other multiples are more likely to be premature than single birth babies.
Very low birth weight is a term used to describe babies who are born weighing less than 3 pounds, 4 ounces. It is very rare that babies are born this tiny. Only about 1 in 100 babies born in the U.S. are very low birth weight.
Low birth weight is a term used to describe babies who are born weighing less than 5 pounds, 8 ounces (2,500 grams). An average newborn usually weighs about 8 pounds. A low-birth-weight baby may be healthy even though they are small. But a low-birth-weight baby can also have many serious health problems.
Small for gestational age is a term used to describe babies that are smaller than normal for the number of weeks of pregnancy. These babies have birth weight below the 10th percentile. This means they are smaller than many other babies of the same gestational age.
Large for gestational age is used to describe newborn babies who weigh more than the usual amount for the number of weeks of pregnancy. Babies are called large for gestational age if they weigh more than 9 in 10 babies of the same gestational age.
The normal length of pregnancy is 37 to 41 weeks. Postmaturity is a word used to describe babies born after 42 weeks. Very few babies are born at 42 weeks or later. Other terms often used to describe these late births include post-term, postmaturity, prolonged pregnancy, and post-dates pregnancy.