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Premature Birth and Postmature Birth
In humans, premature birth refers to any birth that occurs significantly before or after the expected date of delivery.
A premature birth is defined as one that occurs less than 37 weeks after conception. In the United States prematurity occurs in about 7 to 9 percent of pregnancies in white women and about 17 percent in black women. A presumptive reason (usually multiple pregnancy, maternal toxemia or hypertension, abnormal attachment of the placenta, or congenital malformation of the infant) can be found for 40 to 60 percent of premature births. Poor maternal health, hygiene, and nutrition increase the likelihood of prematurity; maternal accidents and acute illness are insignificant as causes. The chief specific causes of death among premature infants are respiratory disturbances, infections, and spontaneous hemorrhages, especially into the brain or lungs. With good care, about 85 percent of all live-born premature infants should survive; those of higher weight have a better chance.
Prematurity is to be distinguished from intrauterine growth retardation, in which weight and development are subnormal for fetal age. An estimated 1.5 to 2 percent of all babies are significantly below a birth weight proper to their fetal age. Deficiency of transplacental nutrition from various causes is frequently responsible. Other causes include fetal infections and some malformations. Generally, babies under 5.5 pounds but carried for more than 37 weeks are considered growth-retarded rather than premature.
A postmature birth is any birth that occurs more than three weeks after the expected date of delivery, at which time placental transfer begins to fail, and the fetus receives decreased amounts of oxygen and nutrients. If birth does not occur naturally or is not induced, the fetus will die. Postmature newborns are often thin, with dry, wrinkled skin and unusually long hair and nails. If the postmature child lives through the first few days after birth, its chances for survival are good.