Worldwide, prematurity causes ten percent of neonatal mortality, or around 500,000 deaths per year. In the U.S., where many infections and other causes of neonatal death have been markedly reduced, prematurity is the leading cause of neonatal mortality at 25%. Prematurely born infants are also at greater risk of developing serious health problems, such as cerebral palsy, chronic lung disease, gastrointestinal problems, mental retardation, vision loss, or hearing loss. They are also more likely to have depression as teenagers.