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Essentials of Diagnosis

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  • Estimated gestational age of greater than 20 0/7 weeks and less than 37 0/7 weeks
  • Regular uterine contractions at frequent intervals
  • Documented cervical change or appreciable cervical dilatation or effacement

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Pathogenesis

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Labor is the process of coordinated uterine contractions leading to progressive cervical effacement and dilatation by which the fetus and placenta are expelled. Preterm labor is defined as labor occurring after 20 weeks' but before 37 weeks' gestation. Although there is no strict definition in the literature regarding the amount of uterine contractions required for preterm labor, there is consensus that contractions need to be regular and at frequent intervals. Generally, more than 4 contractions per hour are needed to cause cervical change. The uterine contractions need not be painful to cause cervical change and may manifest themselves as abdominal tightening, lower back pain, or pelvic pressure. In addition, there must be demonstrated cervical effacement or dilatation to meet a diagnosis of preterm labor.

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It is important to distinguish preterm labor from other similar clinical entities, such as cervical incompetence (cervical change in the absence of uterine contractions) and preterm uterine contractions (regular contractions in the absence of cervical change) because the treatment for these situations differs. Cervical incompetence may require cerclage placement, and preterm uterine contractions without cervical change is generally a self-limited phenomenon that resolves spontaneously and requires no intervention. If ruptured membranes accompany preterm labor, these cases are classified as preterm premature rupture of membranes (for discussion of diagnosis see Premature Rupture of Membranes).

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Preterm birth complicates approximately 12% of all pregnancies in the United States. It is the number one cause of neonatal morbidity and mortality and causes 75% of neonatal deaths that are not due to congenital anomalies.

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Thirteen percent of all infants are classified as low birth weight (<2500 g), of whom 25% are mature low-birth-weight infants and approximately 75% are truly premature. The latter group accounts for nearly two-thirds of infant deaths (approximately 25,000 annually in the United States). Approximately 30% of premature births are due to miscalculation of gestational age or to medical intervention required by the mother or fetus.

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The care of premature infants is costly. Compared with term infants, those born prematurely suffer greatly increased morbidity and mortality (eg, functional disorders, abnormalities of growth and development). Thus every effort is made to prevent or inhibit preterm labor. If preterm labor cannot be inhibited or is best allowed to continue, it should be conducted with the least possible trauma to the mother and infant.

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Many obstetric, medical, and anatomic disorders are associated with preterm labor. Some of the risk factors are listed in Table 14–1. Detailed discussions of these conditions are given in other chapters. The cause of preterm labor in 50% of pregnancies, however, is idiopathic. Although several prospective risk-scoring tools are in use, they have not been convincingly demonstrated to be of value.

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Table Graphic Jump Location
Table 14–1. Risk Factors Associated with Preterm Labor.

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