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Preterm birth: Where we are, where we are going

by Michael O. Gardner, MD, MPH
Associate Professor, Gynecology & Obstetrics

Preterm delivery is an important problem facing obstetricians, neonatologists, and pregnant women and their families. Though less than 12% of U.S. deliveries are preterm births, they are the largest cause of infant mortality not due to congenital malformations and are the main cause of permanent sequelae. Despite much effort, preterm births have increased by almost 20% since the early 1980s, mostly attributable to assisted reproductive technology (ART) and increased risk of multiple gestation.

Practitioners might reasonably assume that the ability to predict which patients are most likely to deliver preterm would enhance the ability to prevent these births. Physicians have tried to make such predictions, including through assessment of epidemiological factors such as a low pre-pregnancy body mass index (<19.6), African-American race, maternal history of hypertension, cigarette smoking, illicit drug use, and, the greatest risk factor, previous history of preterm delivery. Unfortunately, while risk scoring can identify which women have an increased risk of preterm births, proposed interventions to lower the preterm birth rate have not proved successful.

Several biochemical tests have been studied to find an identifier of risk. Positive fibronectin on a cervical swab in a symptomatic woman at 24 weeks’ gestation is associated with 60-fold risk of preterm birth before 37 weeks; but, the positive predictive value of a positive fibronectin is 25–35%. In a woman presenting with contractions, a negative fetal fibronectin has a negative predictive value of 95%.

Research on the role of transcervical ultrasound has shown that a cervical length <25 mm is associated with a greatly elevated risk of preterm delivery. However, results of four randomized trials conflict concerning the efficacy of placing a cervical cerclage to reduce the risk. A large NIH-sponsored multicenter randomized trial is underway to address the issue. Currently, routine cervical cerclage is inappropriate based only on the presence of a shortened cervix.

The only two interventions proved to enhance the outcome of a preterm infant are a full course of antenatal corticosteroids and subsequent delivery in a perinatal center with a neonatal intensive care unit. Hopefully, research into the many potential causes of the preterm birth (eg, intrauterine infection, hormonal mediation of preterm contractions, and the effects of ART) will lead to effective treatment and prevention strategies.