Association between mid-trimester cervical length and risk of spontaneous preterm birth is modified by a prior cervical excisional procedure.

To determine whether and how prior cervical excision modifies the association between a short residual mid-trimester cervical length and the risk of spontaneous preterm birth (SPB). This is a retrospective cohort study on spontaneous preterm birth (SPB) in a total of 1875 patients with and without prior cervical excisional procedure (CEP, loop electrosurgical excision procedure or cold knife cone). Adjusting for potential confounders, we conducted a logistic regression analysis, along with a test of interaction between prior history of CEP and cervical length on SPB. Among 1875 women, there were 274 patients who experienced preterm birth (33.9%). Risk of SPB is significantly associated with age, BMI, cervical length (CL), gestational week at measurement of CL by ultrasonography, multipara, history of preterm birth, in vitro fertilization (IVF) and CEP. Moreover, a test of interaction indicated a close association between risk of SPB and CL, which was modified by history of prior CEP. We identified a CL cutoff value of 2.8 cm at which the risk of SPB in patients with prior CEP was identical to the risk in patients without prior CEP. Given the same CLs which were less than 2.8 cm, prior CEP patients presented a higher risk of preterm birth than non-CEP ones. In contrast, given the same CLs which were more than 2.8 cm, prior CEP patients presented lower risk of preterm birth than non-CEP ones. Association between mid-trimester cervical length and risk of SPB was modified by a prior cervical excisional procedure. In patients with prior CEP compared to those without, there was an increased risk of SPB at CL < 2.8 cm and a decreased risk of SPB at CL > 2.8 cm.