Premature birth (also known as preterm birth) is defined medically as childbirth occurring earlier than 37 completed weeks of gestation. Most pregnancies last about 40 weeks. About 12 percent of babies in the United States - or 1 in 8 - are born prematurely each year. In 2003, more than 490,000 babies in the U.S. were born prematurely. Premature babies are sometimes called preemies. The shorter the term of pregnancy is, the greater the risks of complications. Infants born prematurely have an increased risk of death in the first year of life; prematurity itself is the leading cause of newborn death within one month of birth at 25%. They are also at a greater risk for developing serious health problems such as: cerebral palsy, chronic lung disease, gastrointestinal problems, mental retardation, vision and hearing loss.