long medical complications such as cerebral palsy, visual and hearing disabilities, and mental retardation.

Premature babies often spend months fighting for survival and struggling to overcome illness. The impact on the families is often long-lasting and results in emotional and financial hardships. The enormous medical costs associated with preterm birth, during infancy and often continuing throughout the child’s life, represent about 35 percent of all medical expenditures for newborns and about 10 percent of all medical expenditures for children. Some estimates indicate that in the first year of life alone, a preterm birth costs about $15,000 more than a full-term birth.

Although vast improvements have been made in preventing deaths of premature infants, little success has been attained in understanding and preventing preterm birth, and the knowledge that has been gained about preterm labor has not translated into improved perinatal outcomes. Despite all efforts to reduce the condition, the rate of preterm birth has increased during the past 20 years. Since the early 1980s, the rate of preterm birth in the United States has increased by 17 percent, and the incidence of low birth weight (less than 2,500 grams) has risen 10 percent. For reasons that are not fully understood, these problems take a disproportionate toll on African Americans and recent immigrants from Latin America. Such demographic differences have led many to characterize preterm birth as a disease of poverty.

Attempts have been made to reduce the prevalence of the conditions leading to preterm birth, especially preeclampsia and fetal growth restriction. Interventions such as improved nutrition, maternal bed rest, low-dose aspirin, and calcium supplementation have been tried, but they have generally failed. Evidence suggests that although the prevalence of preeclampsia and fetal growth restriction appears to have remained unchanged, early delivery for fetal distress associated with these conditions has reduced stillbirths.

Clearly, preterm birth is not an acute event. Its roots may well begin before pregnancy, perhaps even in a woman’s early life. The discussion of preterm birth is confused by a lack of firm agreement about, and understanding of, what constitutes “preterm.” The length of the natural term of pregnancy exhibits tremendous variability, and the gestational period for an infant with low birth weight can be within a normal range. Further, the assignment of due dates for delivery is an inaccurate clinical practice. In fact, only 4 percent of all women deliver on their due date.

Normal pregnancy is a carefully programmed sequence of events from the perspective of uterine activity, in which one genetic function leads to another. In general, pregnancy can be divided into four stages. Most of the pregnancy is a stage of uterine quiescence, maintained by active mechanisms. This stage is followed by a stage of preparation or activation, when the contraction-activating proteins are turned on by a variety of mechanisms that prepare the uterus to become a contractile organ. The next stage, onset of labor, requires an integra-



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