Premature birth: A birth that takes place 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. 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.