Each year, nearly 500,000 babies are premature, or preemies. That's 1 of every 8 infants born in the United States. A premature birth is when a baby is born before 37 completed weeks of pregnancy. A full-term pregnancy is 40 weeks.

Important growth and development occur throughout pregnancy - especially in the final months and weeks. Because they are born too early, preemies weigh much less than full-term babies. They may have health problems because their organs did not have enough time to develop. Preemies need special medical care in a neonatal intensive care unit, or NICU. They stay there until their organ systems can work on their own.

Centers for Disease Control and Prevention