A premature baby, or preemie, is born before the 37th week of pregnancy. Premature birth occurs in between 8 percent to 10 percent of all pregnancies in the United States. 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.