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Pregnancy after conization is associated with a high risk of preterm delivery. However, because risk factors for preterm delivery after conization remain unknown, we conducted a multicenter observational study to investigate risk factors associated with preterm delivery. We selected patients who had previously undergone conization and reviewed medical records from 18 hospitals in cooperation with Keio University School of Medicine between January 2013 and December 2019. Women were classified as nulliparous and primiparous, and a multiple logistic regression analysis was performed to evaluate the relative contributions of the various maternal risk factors for preterm delivery (i.e. delivery before 37 gestational weeks). Among 409 pregnant women after conization, 68 women delivered preterm (17%). The incidence of nulliparity (p = .014) was higher and a history of preterm delivery (p = .0010) was more common in the preterm delivery group than in the term delivery group. Furthermore, the proportion of women diagnosed with adenocarcinoma in situ (AIS) and cervical cancer in the preterm delivery group was higher than that in the term delivery group (p = .0099 and .0004, respectively). In multiple regression models in nulliparous women, cervical cancer or AIS (Odds ratio [OR]: 4.16, 95% CI: 1.26–13.68, p = .019) and a short cervix in the second trimester (OR: 13.41, 95% CI: 3.88–46.42, p < .0001) increased the risk of preterm delivery. Furthermore, a history of preterm delivery (OR: 7.35, 95% CI: 1.55–34.86, p = .012), cervical cancer or AIS (OR: 5.07, 95% CI: 1.24–20.73, p = .024), and a short cervix in the second trimester (OR: 4.29, 95% CI: 1.11–16.62, p = .035) increased the risk of preterm delivery in the multiple regression models in primiparous women. Pregnant women who previously underwent conization are at risk for preterm delivery. The histological type of AIS and cervical cancer was evaluated as a risk factor for preterm delivery. KEY MESSAGESPrior preterm delivery, presence of a short cervix, and cervical cancer or AIS were predictors of preterm delivery after conization.The depth of conization in cervical cancer or AIS group was significantly larger than that in the CIN group. Prior preterm delivery, presence of a short cervix, and cervical cancer or AIS were predictors of preterm delivery after conization. The depth of conization in cervical cancer or AIS group was significantly larger than that in the CIN group.