Cervical Length at Mid-Pregnancy and the Risk of Primary Cesarean Delivery

Abstract

Previous research suggests that normal term birth depends on programmed development of the uterus in early pregnancy. A short cervix in mid-pregnancy is associated with an increased risk of spontaneous preterm birth. The present study tested the hypothesis that a long cervix in mid-pregnancy is associated with a higher risk of cesarean delivery during term labor. The study population included 27,472 primiparous women whose cervical length was 16 mm or more at a median gestational age of 23 weeks, and who delivered a live infant in labor at term. Cervical length was estimated by transvaginal ultrasonography. The rate of cesarean delivery at term was lowest (16%) in women whose mid-pregnancy cervical length was in the lowest quartile (16–30 mm). The rate was progressively greater in the second quartile (18%, 31–35 mm), third quartile (22%, 36–39 mm), and fourth quartile (26%, 40–67 mm). Compared with women in the first quartile, the odds ratio (OR) for cesarean delivery in women in the fourth quartile was 1.8 (95% confidence interval [CI], 1.66–1.97). The OR was 1.68 (95% CI, 1.53–1.84) after adjusting for numerous variables including maternal age, body mass index, smoking status, race or ethnic group, gestational age at birth, spontaneous or induced labor, and birth weight percentile. The association with cervical length was slightly stronger when the outcome was limited to operative delivery mandated by the failure of labor to progress. An increased cervical length during mid-pregnancy appears to independently predict the risk of cesarean delivery during labor at term in primiparous women. This finding suggests that poor progress during labor in women who deliver at term may reflect dysfunctional development of the uterus at earlier stages of pregnancy.