The use of two measures, fetal fibronectin (a protein) levels and transvaginal cervical length, had low predictive accuracy for spontaneous preterm birth among women who have not given birth before, according to a study appearing in the March 14 issue of JAMA.
Preterm birth, affecting approximately 1.2 percent of the deliveries in the United States, was responsible for 35 percent of the world’s 3.1 million annual neonatal deaths in 2006. Current strategies to identify women at risk are largely based on prior pregnancy outcomes, but risk assessment in women pregnant for the first time is difficult. The combination of transvaginal cervical length and fetal fibronectin levels to identify women at risk has been studied, with conflicting results.
Sean Esplin, M.D., of Intermountain Healthcare, Salt Lake City, and colleagues conducted a study that included 9,410 women without prior childbirth who had transvaginal cervical length and vaginal fetal fibronectin levels reviewed at two study visits four or more weeks apart.
Among these women, 474 (5 percent) had spontaneous preterm births, 335 (3.6 percent) had medically indicated preterm births, and 8,601 (91 percent) had term births. The researchers found that fetal fibronectin levels and transvaginal cervical length had poor predictive performance as screening tests for spontaneous preterm birth before 37 weeks. The most commonly used clinical cutoff for transvaginal cervical length (threshold of 25 mm or less) identified a minority (23 percent) of spontaneous preterm births before 37 weeks. The addition of fetal fibronectin levels to transvaginal cervical length measurement did not increase the predictive performance of transvaginal cervical length alone. Fetal fibronectin levels of 50 ng/mL or greater at 16 to 22 weeks identified 30 of 410 women (7.3 percent) with spontaneous preterm birth and 31 of 384 (8.1 percent) at 22 to 30 weeks.