The risk of preterm birth in women with history of short cervix delivering at term in the previous pregnancy: a retrospective cohort study.

PURPOSE: To investigate whether women with a short cervical length (CL), but delivered at term in the first pregnancy might have increased risks of having short cervix and spontaneous preterm birth (sPTB) in the subsequent pregnancies.

METHODS: This is a retrospective cohort study including singleton gestations who were delivered between Jan 2011 and Dec 2018, who had had one childbirth experience and who had transvaginal sonographic CL assessment performed at mid-trimester (18 ~ 30 weeks) in both pregnancy. The women were divided into four group according to the history of preterm birth and a short cervix (< 25 mm before 30 weeks of gestation): (1) the Preterm-short cervix group, (2) the Preterm-no shortening group, (3) the Term-short cervix group, and (4) the Term-no shortening group. We compared the risk of having short cervix and sPTB during the second pregnancy of women. Secondary outcomes were threatened preterm labor, need for tocolytics, and cerclage placement.

RESULTS: A total of 804 women met our inclusion criteria. The rate of having short cervix (< 25 mm before 28 weeks of gestation) during the second pregnancy in women in the Term-short cervix group (43.2%) was significantly higher than those in women in the Term-no shortening group (6.6%), and in the Preterm-no shortening group (8.8%) (all p < 0.001 with Bonferroni correction), but not higher than those in women with the Preterm-short cervix group (30.8%, p > 0.05 with Bonferroni correction). When compared with women in the Preterm-no shortening group, women in the Term-short cervix group were also at an increased risk of need for need of tocolytics (60.2% vs. 26.5%) and cerclage placement (15.9% vs. 6.1%, all p < 0.001). Women in the Term-short cervix group had an increased risk of sPTB (< 37 weeks) during the second pregnancy, as compared to those in the Term-no shortening group (adjusted odds ratio 5.098, 95% CI 2.107-11.874).

CONCLUSION: Women with a history of short cervix in their first pregnancy, but who delivered at term, were at increased risk of having a short cervix and sPTB in their second pregnancy, as compared to women with a history of term delivery without cervical shortening. Thus, short cervix of the previous pregnancy might be a predictive factors for preterm birth in the subsequent pregnancy.