Rates of preterm birth in the United States have been falling since 2006, but the rates of early preterm birth in singletons (those under 34 weeks’ gestation), specifically, have not trended downward as dramatically as have late preterm birth in singletons (34-36 weeks). According to 2015 data from the National Vital Statistics Reports, the rate of early preterm births is still 3.4% in all pregnancies and 2.7% among singletons.
While the number of neonates born before 37 weeks of gestation remains high – approximately 11% in 2013 – and signifies a continuing public health problem, the rate of early preterm birth is particularly concerning because early preterm birth is more significantly associated with neonatal mortality, long-term morbidity and extended neonatal intensive care unit stays, all leading to increased health care expenditures.
Finding predictors for preterm birth that are stronger than traditional clinical factors has long been a goal of ob.gyns. because the vast majority of all spontaneous preterm births occur to women without known risk factors (i.e., multiple gestations or prior preterm birth).
Cervical length in the midtrimester is now a well-verified predictor of preterm birth, for both low- and high-risk women. Furthermore, vaginal progesterone has been shown to be a safe and beneficial intervention for women with no known risk factors who are diagnosed with a shortened cervical length (< 2 cm), and cervical cerclage has been suggested to reduce the risk of preterm birth for women with a history of prior preterm birth who also have a shortened cervical length.
Some are now advocating universal cervical length screening for women with singleton gestations, but before universal screening is mandated, the downstream effect of such a change in practice must be considered.
Backdrop to screening
Cervical length measurement was first investigated more than 25 years ago as a possible predictor of preterm birth. In 1996, a prospective multicenter study of almost 3,000 women with singleton pregnancies showed that the risk of preterm delivery is inversely and directly related to the length of the cervix, as measured with vaginal ultrasonography (N. Engl. J. Med. 1996;334:567-72).
In fact, at 24 weeks’ gestation, every 1 mm of additional cervical length equates to a significant decrease in preterm birth risk (odds ratio, 0.91). Several other studies, in addition to the landmark 1996 study, have similarly demonstrated this inverse relationship between preterm birth risk and cervical length between 18 and 24 weeks’ gestation.
However, the use of cervical measurement did not achieve widespread use until more than a decade later, when researchers began to identify interventions that could prolong pregnancy if a short cervix was diagnosed in the second trimester.
For example, Dr. E.B. Fonseca’s study of almost 25,000 asymptomatic pregnant women, demonstrated that daily vaginal progesterone reduced the risk of spontaneous delivery before 34 weeks by approximately 44% in women identified with a cervical length of 1.5 cm or less (N. Engl. J. Med. 2007;357:462-9). The vast majority of the women in this study had singleton pregnancies.
Shortly thereafter, Dr. S.S. Hassan and her colleagues completed a similar trial in women with singleton gestations and transvaginal cervical lengths between 1.0 and 2.0 cm at 20-23 weeks’ gestation. In this trial, nightly progesterone gel (with 90 mg progesterone per application) was associated with a 45% reduction in preterm birth before 33 weeks and a 38% reduction in preterm birth before 35 weeks (Ultrasound. Obstet. Gynecol. 2011;38:18-31).
A meta-analysis led by Dr. Roberto Romero, which included the Fonseca and Hassan trials, looked specifically at 775 women with a midtrimester cervical length of 2.5 cm or less. Women with a singleton gestation who had no history of preterm birth had a 40% reduction in the rate of early preterm birth when they were treated with vaginal progesterone (Am. J. Obstet. Gynecol. 2012;206:124-e1-19).
The benefits of identifying a short cervix likely extend to women with a history of prior preterm birth. A patient-level meta-analysis published in 2011 demonstrated that cervical cerclage placement was associated with a significant reduction in preterm birth before 35 weeks’ gestation in women with singleton gestations, previous spontaneous preterm birth, and cervical length less than 2.5 cm before 24 weeks’ gestation (Obstet. Gynecol. 2011;117:663-71).
The possible benefits of diagnosing and intervening for a shortened cervix have tipped many experts and clinicians toward the practice of universal cervical length screening of all singleton pregnancies. Research has shown that we can accurately obtain a cervical-length measurement before 24 weeks, and that we have effective and safe interventions for cases of short cervix: cerclage in women with a history of preterm birth who are already receiving progesterone, and vaginal progesterone in women without such a history.