THE LENGTH OF THE CERVIX AND THE RISK OF SPONTANEOUS PREMATURE DELIVERY

Background. The role of the cervix in the pathogenesis of premature de livery is controversial. In a prospective, multicenter study of pregna nt women, we used vaginal ultrasonography to measure the length of the cervix; we also documented the incidence of spontaneous delivery befo re 35 weeks' gestation. Methods. At 10 university-affiliated prenatal clinics, we performed vaginal ultrasonography at approximately 24 and 28 weeks of gestation in women with singleton pregnancies. We then ass essed the relation between the length of the cervix and the risk of sp ontaneous preterm delivery. Results. We examined 2915 women at approxi mately 24 weeks of gestation and 2531 of these women again at approxim ately 28 weeks. Spontaneous preterm delivery (at less than 35 weeks) o ccurred in 126 of the women (4.3 percent) examined at 24 weeks. The le ngth of the cervix was normally distributed at 24 and 28 weeks (mean [ +/-SD], 35.2+/-8.3 mm and 33.7+/-8.5 mm, respectively). The relative r isk of preterm delivery increased as the length of the cervix decrease d. When women with shorter cervixes at 24 weeks were compared with wom en with values above the 75th percentile, the relative risks of preter m delivery among the women with shorter cervixes were as follows: 1.98 for cervical lengths at or below the 75th percentile (40 mm), 2.35 fo r lengths at or below the 50th percentile (35 mm), 3.79 for lengths at or below the 25th percentile (30 mm), 6.19 for lengths at or below th e 10th percentile (26 mm), 9.49 for lengths at or below the 5th percen tile (22 mm), and 13.99 for lengths at or below the Ist percentile (13 mm) (P<0.001 for values at or below the 50th percentile; P=0.008 for values at or below the 75th percentile). For the lengths measured at 2 8 weeks, the corresponding relative risks were 2.80, 3.52, 5.39, 9.57, 13.88, and 24.94 (P<0.001 for values at or below the 50th percentile; P=0.003 for values at the 75th percentile). Conclusions. The risk of spontaneous preterm delivery is increased in women who are found to ha ve a short cervix by vaginal ultrasonography during pregnancy.