Cervical length indicates risk of preterm delivery [Classics Series]

1. Transvaginal cervical length measured at 24 and 28 weeks gestation was inversely associated with risk of spontaneous preterm delivery in a dose-dependent manner.

Original Date of Publication: February 1996

Study Rundown: Spontaneous preterm delivery (PTD) is one of the most vexing problems in clinical obstetrics. Currently, over 11% of all deliveries in the United States are complicated by PTD. Preterm delivery is the leading cause of neonatal morbidity and mortality and can cause complicated and hard to treat cardiorespiratory, neurologic, opthalmalogic and gastrointestinal problems. Although a prior pregnancy complicated by spontaneous PTD was a known, strong risk factor for a subsequent spontaneous PTD, risk factors for the initial occurrence of spontaneous PTD remained poorly characterized as of the 1990s. For years, clinicians suspected that shorter cervical lengths were associated with preterm parturition, or the premature, inappropriate softening and shortening of the gravid cervix. However, the long-time standard for cervical length measurement, the digital cervical exam, was imprecise and varied greatly between observers. As transvaginal ultrasound evolved in the late half of the 20th century to become part of standard prenatal care, sonographic cervical length measurement emerged as a standardized, reliable and reproducible method of measuring cervical length. The present work represents the largest and most well designed multi-center prospective investigation of the association of cervical length and incidence of spontaneous preterm delivery.

This landmark study demonstrated that cervical length is an indirect indicator of cervical competence and is inversely proportional to spontaneous PTD incidence. Strengths of this investigation included multi-center study, large population-based sample, and enrollment of women who had ultrasound-confirmed, healthy singleton pregnancies. Additionally, transvaginal assessment of cervical length, as opposed to transabdominal ultrasound, improved accuracy and a program of operator training and quality control minimized variability in measurements to the greatest extent possible. Limitations included definition of preterm delivery as delivery at less than 35 weeks, which differs from the current definition of delivery at less than 37 weeks. Inclusion of women with a history of prior PTD, who comprised 16% of the sample, introduced the opportunity for nonrandom misclassification and confounding since women with a history of PTD are more likely to have a short cervix and to experience PTD. Future investigations would go on to reproduce these findings in both primigravid populations and women with a history of prior spontaneous PTD.

Click to read the study in NEJM

Dr. Alan Peaceman, MD, talks to 2 Minute Medicine: Northwestern University School of Medicine; Chief, Division of Obstetrics and Gynecology-Maternal Fetal Medicine.

“This multi-center prospective study demonstrated that short cervical length at 24 and 28 weeks gestation is associated with increased risk of spontaneous preterm delivery such that cervical length can serve as an indirect indicator of cervical competency. Future investigations have reproduced the findings presented herein and demonstrate the inverse association with second trimester cervical length and risk of preterm delivery.”

In-Depth [prospective cohort study]: A total of 2915 women with singleton pregnancies were enrolled across ten university-affiliated clinics in this study conducted by the Maternal Fetal Medicine Network from 1992 through 1994. Cervical length was measured by transvaginal ultrasound at 24 and 28 weeks gestation and the incidence of spontaneous preterm delivery was assessed. Spontaneous PTD was defined as delivery at a gestational age less than 35 weeks. Women with cervical lengths >75th percentile were considered the referent group.

At 24 weeks gestation, women with cervical lengths ≤75th percentile were twice as likely to experience PTD (RR: 1.98, p < 0.01) compared to the referent group. At 28 weeks, cervical lengths ≤75th percentile were associated with twice the risk of PTD (RR: 2.80, p < 0.003) and lengths ≤50th percentile were associated with even greater risk (RR: 3.52, p < 0.001). A dose-response relationship was noted whereby incidence of PTD increased a function of cervical length: ≤25th percentile (RR: 5.4), ≤10th percentile (RR: 9.6), ≤5th percentile (RR: 13.9), ≤1st percentile (RR: 24.9)

Image: PD

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