Preterm birth, also known as premature birth, is the birth of a baby at fewer than 37 weeks' gestational age, as opposed to the usual about 40 weeks. These babies are known as preemies or premmies. Symptoms of preterm labor include uterine contractions which occur more often than every ten minutes or the leaking of fluid from the vagina. Premature infants are at greater risk for cerebral palsy, delays in development, hearing problems and sight problems. The earlier a baby is born, the greater these risks will be. The cause of preterm birth is often not known. Risk factors include diabetes, high blood pressure, being pregnant with more than one baby, being either obese or underweight, a number of vaginal infections, air pollution including tobacco smoking, and psychological stress. It is recommended that labor not be medically induced before 39 weeks unless required for other medical reasons. The same recommendation applies to cesarean section. Medical reasons for early delivery include preeclampsia.