Preterm birth (PTB), also known as premature birth, is the birth of a baby at fewer than 37 weeks’ gestational age, as opposed to full term delivery at approximately 40 weeks. PTB is defined as birth before 37 weeks gestation, very early PTB is before 32 weeks, early PTB occurs around 32–36 weeks, late PTB is between 34–36 weeks gestation and early term birth is delivered at 37–38 weeks gestation  Late PTB accounts for 75% of all PTB.
These babies are known as premature babies or colloquially preemies (American English) or premmies (Australian English). 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, multiple gestation (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.