Preterm labor

Published on 10/03/2015 by admin

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Last modified 10/03/2015

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Chapter 33 PRETERM LABOR

Preterm labor is the leading cause of perinatal morbidity and mortality in the United States. Preterm labor is characterized by increased uterine irritability and cervical effacement or dilatation, or both, before 37 weeks of pregnancy. Preterm labor usually results in preterm birth, which affects 8% to 10% of births in the United States. Unfortunately, in most cases, the precise causes of preterm labor are not known. Risk factors associated with preterm labor are outlined later in this chapter. Women with a history of preterm delivery have the highest risk of recurrence, estimated to be between 17% and 37%. Approximately 40% of spontaneous births following preterm labor are thought to be caused by infection. Screening and treatment for asymptomatic bacteriuria early in pregnancy, which prevents pyelonephritis during pregnancy, has helped reduce the rate of preterm delivery.

The identification of women with preterm contractions who will actually deliver before term is an inexact process. The inability to distinguish accurately between women in “true” preterm labor and those in “false” labor has hampered the assessment of therapeutic interventions, inasmuch as up to 50% of untreated patients do not actually deliver before term.

Several indicators have been studied as means of identifying women who will develop preterm labor and women who will deliver before term. Unfortunately, these indicators are imprecise markers of an activated process of parturition. Risk-scoring strategies have proved to be of limited value. Likewise, regular cervical assessment was found to have no predictive advantage in pregnant women with no predisposing risk factors. Although several studies have demonstrated an inverse relationship between cervix length and frequency of preterm delivery, transvaginal ultrasonography currently is not indicated in the routine evaluation of the patient with a history of or current risk factors for preterm delivery. Home monitoring of uterine activity may identify preterm contractions, but it is not clear whether the use of this system can affect the rate of preterm delivery.

Fetal fibronectin in cervical and vaginal secretions may be a biochemical marker for preterm labor. The presence of fetal fibronectin in the cervix or vagina is infrequent after the 20th week of gestation and rare after the 24th week. After the 24th week, the presence of fetal fibronectin may indicate detachment of the fetal membranes from the deciduas. Studies suggest that fetal fibronectin is a biochemical marker for labor. However, there is no evidence to suggest that the use of the assay for fetal fibronectin would result in a reduction in spontaneous preterm birth. Many questions still remain as to how to use the results of fetal fibronectin assays, both positive and negative, in clinical care.