Preterm Labor Prediction: Singleton and Twin Gestations
This chapter reviews recent studies addressing the prediction of and treatment for spontaneous labor in singletons and twins.
Triage Clinic for Preterm Labor Singletons
Triage clinics for preterm labor prediction identify women with the highest risk for adverse outcome.  Screening was carried out with quantitative fetal fibronectin (qfFN), cervical length, and risk of spontaneous preterm birth. A large prospective cohort of 1130 asymptomatic singleton pregnancies at high risk for preterm birth were screened between 23 and 28 weeks gestation. Six percent of asymptomatic women were correctly identified as at risk for preterm delivery, were admitted to the hospital for observation and were at the highest risk for poor outcome, while the 94% were identified as low-risk asymptomatic outpatient women and were correctly predicted to have better outcomes.
Optimal Cervical Length for Ultrasound Indicated Cerclage
In another study  addressing the optimal cervical length for placing an ultrasound indicated cerclage, the authors assessed a group of women with a previous spontaneous preterm birth. The effectiveness of ultrasound indicated cerclage in these women varied by cervical length cutoff and by the preterm birth gestational age of interest. At cervical length cutoffs during mid-trimester ultrasound of between < 10 mm and < 25 mm, the risk of preterm birth was reduced at < 35 and < 32 weeks. For preterm birth at < 24 weeks, no benefit was noted for cutoffs between < 10 mm and < 15 mm, possibly due to sample size and number.
Cervical Length and Twin Gestations
In a study of twin gestation, the predictive value of cervical length for spontaneous preterm birth at < 32 weeks was addressed.  A total of 980 twin pregnancies were assessed by a cervical length obtained at 18 to 23 weeks gestation. Cervical length was found to be a poor predictor of preterm birth at < 32 weeks in asymptomatic twin pregnancies.
Pessary Use in Twin Gestations
Use of a pessary in twin gestations was addressed in a study to guide the decision for placement in a twin gestation. In a multi-center randomized trial, 808 women were randomized to pessary or control.  In this model, short cervix (< 38 mm), monochorionic twins, and nulliparity were predictive factors for pessary benefit while history of preterm birth and triplets were predictive factors for possible pessary harm. The authors state that pessary use to prevent preterm labor in twins should not be used until the benefit is further validated.
In an editorial review of cervical length as a predictor for spontaneous preterm birth in high-risk singleton pregnancies, the authors make the following general points.  The pathogenesis of premature birth is not fully understood, there are multiple potential pathways without a single endpoint, and there may be different risks and initiating factors for preterm birth. The prior research is based upon heterogeneous populations with differing risk attributes. Therefore, a single parameter such as cervical length, while important, may not serve as the definitive predictor for preterm birth. The authors suggest further studies are needed to define responsible pathways.
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