Premature baby: A baby born before 37 weeks of gestation have passed. Historically, the definition of prematurity was 2500 grams (about 5 1/2 pounds) or less at birth. The current World Health Organization definition of prematurity is a baby born before 37 weeks of gestation, counting from the first day of the Last Menstrual Period (the LMP).
Premature birth carries greater risks the earlier it occurs before that 37-week goal. A baby born a little prematurely may suffer few if any consequences. The earlier in fetal growth and development that birth takes place, the greater is the likelihood that life-support systems will be needed, and the greater the risk for birth defects and death.
Many procedures are available to prevent early birth, from bed rest to medications. If premature birth is medically necessary or inevitable, however, it may be accomplished via C-section to limit stress on the fetus and protect it from damage.
Colloquially, a premature baby is commonly called a preemie.