Timing of mid-trimester cervical length (CL) shortening in high-risk women

Abstract

Prior spontaneous preterm birth (SPTB) and CL shortening are well-described risk factors for recurrent SPTB, but the mechanisms that lead to cervical change and SPTB are poorly understood. Consenting high-risk women, defined as at least 1 prior SPTB<34 wks, were recruited at 12 US centers and underwent q2 wk vaginal sonographic evaluations beginning at 16-19 wks and continued until either 22 6/7 wks or a CL<25 mm. Fundal pressure-induced and spontaneous CL shortening were used to define the (shortest) CL at each scan. Survival curves were used to compare the gestational age (GA) at CL shortening <25 mm in women whose earliest prior SPTB was <27 weeks versus 27-33 weeks' gestation. 998 scans were performed on 367 women, 36% of whom developed a CL<25 mm. Of the 193 in the <27 wk group, 44% experienced CL shortening versus 28% in the 27-33 wk group (p = .002). The Kaplan-Meier survival curves below depict the natural history of CL shortening and demonstrate a significant difference (p = .001) in GA at CL shortening between the 2 groups. A Cox proportional-hazard model confirmed a hazard ratio of 1.8 (p = .001) for cervical shortening in the <27 wk group. Women with a prior SPTB <34 weeks have an appreciable rate of mid-trimester CL shortening in a subsequent pregnancy. However, the observation that nearly 2/3 of them do not shorten their cervix by 22 6/7 weeks indicates significant biologic variation. Women with a prior SPTB <27 weeks shorten their cervix significantly more often and at a significantly earlier gestational age than women whose prior SPTB occurred at 27-33 weeks.