Preterm labor by definition occurs between 20 and 37 weeks of gestation and is the most common com-plication of the third trimester. Approximately 8% of live-born infants in the United States are deliv-ered before term. Important contributory maternal factors include extremes of age, inadequate prenatal care, unusual body habitus, increased physical activ-ity, infections, prior preterm labor, multiple gesta-tion, and other medical illnesses or complications during pregnancy.
Because of their small size and incomplete development, preterm infants—particularly those less than 30 weeks of gestational age or weighing less than 1500 g—experience a greater number of complications than term infants. Premature rupture of membranes complicates one third of premature deliveries; the combination of premature rupture of membranes and premature labor increases the like-lihood of umbilical cord compression resulting in fetal hypoxemia and asphyxia. Preterm infants with a breech presentation are particularly prone to pro-lapse of the umbilical cord during labor. Moreover, inadequate production of pulmonary surfactant fre-quently leads to the idiopathic respiratory distress syndrome (hyaline membrane disease) after deliv-ery. Surfactant levels are generally adequate only after week 35 of gestation. Lastly, a soft, poorly calci-fied cranium predisposes these neonates to intracra-nial hemorrhage during vaginal delivery.
When preterm labor occurs before 35 weeks of gestation, bed rest and tocolytic therapy are usually initiated. Treatment is successful in 75% of patients. Labor is inhibited until the lungs mature and suffi-cient pulmonary surfactant is produced, as judged by amniocentesis. The risk of respiratory distress syndrome is markedly reduced when the amniotic fluid lecithin/sphingomyelin ratio is greater than 2. Glucocorticoid (betamethasone) may be given to induce production of pulmonary surfactant, which requires a minimum of 24–48 h. The most com-monly used tocolytics are β2-adrenergic agonists (ritodrine or terbutaline) and magnesium (6 g intra-venously over 30 min followed by 2–4 g/h). Ritodrine (given intravenously as 100–350 mcg/min) and ter-butaline (given orally as 2.5–5 mg every 4–6 h) also have some β1-adrenergic receptor activity, which accounts for some of their side effects. Maternal side effects include tachycardia, arrhythmias, myo-cardial ischemia, mild hypotension, hyperglycemia, hypokalemia, and, rarely, pulmonary edema. Other tocolytic agents include calcium channel blockers (nifedipine), prostaglandin synthetase inhibitors,oxytocin antagonists (atosiban), and possibly nitric oxide. Fetal ductal constriction can occur after 32 weeks of gestation with nonsteroidal antiinflam-matory drugs such as indomethacin, but it is usually transient and resolves after discontinuation of the drug; renal impairment in the fetus may also cause oligohydramnios.
When tocolytic therapy fails to arrest labor, anesthesia often becomes necessary. The goal dur-ing vaginal delivery of a preterm fetus is a slow con-trolled delivery with minimal pushing by the mother. An episiotomy and low forceps are often employed. Spinal or epidural anesthesia allows complete pel-vic relaxation. Cesarean section is performed for fetal distress, breech presentation, intrauterine growth retardation, or failure of labor to progress. Residual effects from β-adrenergic agonists may complicate general anesthesia. The half-life of rito-drine may be as long as 3 h. Ketamine and ephed-rine (and halothane) should be used cautiously due to interaction with tocolytics. Hypokalemia is usu-ally due to an intracellular uptake of potassium and rarely requires treatment; however, it may increase sensitivity to muscle relaxants. Magnesium therapy potentiates muscle relaxants and may predispose to hypotension (secondary to vasodilation). Residual effects from tocolytics interfere with uterine con-traction following delivery. Lastly, preterm new-borns are often depressed at delivery and frequently need resuscitation. Preparations for resuscitation should be completed prior to delivery.