Prematurity is defined as a birth that occurs before 37 completed weeks (less than 259 days) of gestation. It is associated with approximately one-third of all infant deaths in the United States. Infants born at or before 25 weeks of gestation have the highest mortality rate (about 50 percent) and if they survive, are at the greatest risk for severe impairment .
The incidence and mortality rate of preterm birth will be reviewed here. The risk and pathogenesis of preterm birth and complications of prematurity are discussed separately. (See "Risk factors for preterm labor and delivery" and "Pathogenesis of spontaneous preterm birth" and "Short-term complications of the premature infant".)
Different degrees of prematurity are defined by gestational age (GA), which is calculated from the first day of the mother's last period, or birthweight (BW).
One classification based upon BW includes the following categories (table 1):
- Low birth weight (LBW) – BW less than 2500 g
- Very low birth weight (VLBW) – BW less than 1500 g
- Extremely low birth weight (ELBW) – BW less than 1000 g