Premature birth (also known as preterm birth) is defined medically as birth occurring earlier than 37 completed weeks of gestation. Most pregnancies last about 40 weeks. Premature babies are sometimes called preemies. About 12 percent of babies in the United States - or 1 in 8 - are born prematurely each year. In 2002, more than 480,000 babies in the U.S. were born prematurely. The shorter the term of pregnancy is, the greater the risks of complications. ...