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PRETERM LABOR

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Introduction

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Prematurity is a serious cause of adverse perinatal outcome. It is a contributing factor in 75% of neonatal deaths and may result in neonatal neurologic injury.1 Preterm labor is defined as the onset of labor prior to 37 weeks’ gestation. Anesthetic management includes analgesia for labor and vaginal delivery, as well as anesthesia for cesarean delivery if indicated. Furthermore, the anesthesiologist may become involved in managing the effects of tocolytic agents.

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Risk Factors for Preterm Labor

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Genetic, hormonal, psychosocial, and environmental factors are believed to be associated with preterm labor. Risk factors, identified in fewer than 50% of cases, include, but are not limited to nonwhite race, low socioeconomic status, history of preterm delivery, multiple gestation, preterm premature rupture of membranes, abnormal uterine anatomy, abnormal cervical anatomy, genital or systemic infection, trauma, abdominal surgery, fetal genetic abnormalities, fetal death, and tobacco/substance use.2

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Pathophysiology of Preterm Labor

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Initiation of labor is complex and multifactorial, and it includes genetic as well as hormonal factors. The characteristics of both term and preterm labor include cervical dilation and effacement, increased uterine contractility, and activation of the amniochorionic membrane. In term gestation, these changes are routine conclusions to pregnancy. However, in preterm labor, these changes are initiated through pathologic mechanisms.3 During normal pregnancy, activation of the fetal hypothalamic-pituitary-adrenal axis contributes to labor initiation. Secretion of adrenocorticotropic hormone (ACTH) increases in response to release of corticotropin-releasing hormone from the hypothalamus. ACTH, in turn stimulates the adrenal glands to secrete cortisol.4 This leads to an inflammatory response resulting in increased myometrial prostaglandin, which in turn produces an increase in intracellular calcium, with subsequent initiation of uterine contractions.2 Thus, any stimulus that can initiate the inflammatory cascade of mediators can lead to uterine contractions, even prior to term of pregnancy.

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Diagnosis

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Preterm labor occurs at 20 to 37 weeks’ gestation. Uterine contraction frequency should be at least 4 or more in a 20-minute period, or 8 or more in a 60-minute period. There must be either ongoing cervical changes, cervical dilation of at least 2 cm, or effacement of at least 80%.2 False labor is characterized by irregular contractions that do not increase in frequency, duration, or strength. Also, there is no change in cervical dilation or effacement with false labor.3

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Tocolytics versus Delivery

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Not all patients with preterm labor progress to preterm delivery. Preterm delivery is associated with the potential for increased neonatal morbidity and mortality; therefore, prevention of delivery, or at least prolongation of pregnancy, is desirable. Tocolytic medications inhibit uterine contractions and should be considered for parturients at 20 to 34 weeks’ gestation with reassuring fetal status and absence of infection2 who are in preterm labor. Contraindications to tocolysis include fetal death, fetal anomalies incompatible ...

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