Some evidence supports the concept that cervical incompetence and preterm labor lie along a spectrum leading to preterm delivery. Consequently, investigators have evaluated cerclage placement after preterm labor begins to manifest clinically. Althuisius and colleagues (2003) randomly assigned 23 women with cervical incompetence before 27 weeks to bed rest, with or without emergency McDonald cerclage. Delivery delay was significantly greater in the cerclage group compared with those assigned to bed rest-54 versus 24 days, respectively. Terkildsen and coworkers (2003) studied 116 women who underwent second-trimester emergency cerclage. Nulliparity, membranes extending beyond the external cervical os, and cerclage before 22 weeks were associated with a signiicantly decreased chance of signiicant pregnancy continuation. For women facing a poor pregnancy prognosis due to cervical dilation at midgestation, it seems reasonable to ofer emergency or rescue cerclage with appropriate counseling. However, it is unclear if such interventions truly confer a beneit or merely increase the risk of membrane rupture and infection (Hawkins, 2017).
• Tocolysis to Treat Preterm Labor
Although several drugs and other interventions have been used to prevent or inhibit preterm labor, none is completely efective. The American College of Obstetricians and Gynecologists (2016b) has concluded that tocolytic agents do not markedly prolong gestation but may delay delivery in some women for up to 48 hours. This may allow transport to an obstetrical center with higher-level neonatal care and permit time for a course of corticosteroid therapy. Although delivery may be delayed to administer corticosteroids, treatment has not improved perinatal outcome rates (Gyetvai, 1999).
Beta-adrenergic agonists, magnesium sulfate, calciumchannel blockers, or indomethacin are the recommended tocolytic agents for this short-term use. The gestational age range for tocolytic use is debatable. But, because corticosteroids are not generally used after 34 weeks, and because the perinatal outcomes in preterm neonates are generally good after this time, most do not recommend use of tocolytics after 33 weeks' gestation (Goldenberg, 2002).
In many women, tocolytics stop contractions temporarily but rarely prevent preterm birth. he College (2016b) notes that maintenance therapy with tocolytics is inefective for preventing preterm birth. Importantly, no trial has ever convincingly shown reductions in rates of any important adverse outcome by a tocolytic drug compared with placebo (Walker, 2016). Maintenance tocolysis after acute therapy is not recommended.
Several compounds react with 3-adrenergic receptors to reduce intracellular ionized calcium levels and prevent activation of myometrial contractile proteins (Chap. 21, p. 406). Of 3-mimetic drugs in the United States, ritodrine and terbutaline have been used in obstetrics, but only ritodrine is approved for preterm labor by the FDA.
Ritodrine was voluntarily withdrawn from the United States market in 2003, but a discussion of ritodrine is included here to present issues with 3-mimetic drug use. In one early trial, neonates whose mothers were treated with ritodrine for threatened preterm labor had lower rates of preterm birth and its complications (NIerkatz, 1980). In a randomized trial at Parkland Hospital, intravenous ritodrine delayed delivery for 24 hours but without other beneits (Leveno, 1986b). Additional studies confirmed a delivery delay up to 48 hours (Canadian Preterm Labor Investigators Group, 1992).