Preterm birth and mid-trimester cervical length: Cervical competence as a continuum

Abstract

Prior reports suggest that cervical competence is best conceptualized as a biologic continuum. To test this hypothesis, we studied the relationship between past obstetric performance and mid-trimester cervical length (CL). Consenting women who had at least one prior spontaneous preterm birth (SPTB) <34 weeks were recruited at 12 US centers for a randomized intervention trial. Biweekly vaginal scans were scheduled, beginning at 16-19 weeks' gestation, and continued until the earlier of 22 6/7 weeks or a CL<25 mm (the 10th percentile in a similar population). Fundal pressure-induced and spontaneous dynamic cervical shortening was assessed and used to determine the shortest observed CL at each scan. ANOVA was used to model the relationship between shortest CL observed on serial scans and the gestational age (GA) grouping of each patient´s most recent birth (GArecent) and also her earliest prior SPTB (GAearly). The study population comprised 359 women whose GArecent median (range) was 28.5 (17, 42) weeks and their GAearly was 26 (17, 33) weeks. Their median shortest CL was 29 (0, 60) mm. In 241 (67%) cases, the patient´s GArecent was also her GAearly. The relationship between GArecent and CL was significant (p<.0001): women (N = 53) whose GArecent was <20 weeks had a mean (SD) CL of 24.3 (9.4) mm, versus 26.7 (9.5) mm if GArecent was 21-26 weeks (N = 92), 29.3 (9.9) mm if 27-32 weeks (N = 103), 29.9 (11) mm if 33-35 weeks (N = 27), and 33 (9.1) mm if >35 weeks (N = 84). The relationship between GAearly and CL was also significant (p = .04) but did not follow the same progressive linear trend for CL across all GA groups as did GArecent. Nevertheless, women whose GAearly was <20 weeks still had the shortest mid-trimester CL: 27.2 (11) mm of all the GA groups. There is a highly significant relationship between the GA of the most recent birth and mid-trimester CL in the next pregnancy. These data support CL as a reasonable surrogate for cervical competence and that cervical competence functions along a biologic continuum of obstetric performance.