Risk factors for perineal and vaginal tears in primiparous women – a prospective cohort study

BackgroundThe aim of this study was to estimate the incidence of second-degree perineal tears, obstetric anal sphincter injuries, and high vaginal tears in primiparous women, and to examine how sociodemographic and pregnancy characteristics, hereditary factors, obstetric management and the delivery process are associated with the incidence of these tears.MethodsAll nulliparous women registering at the maternity health care in Region Örebro County, Sweden, in early pregnancy between 1 October 2014 and 1 October 2017 were invited to participate in a prospective cohort study. Data on maternal and obstetric characteristics were extracted from questionnaires completed in early and late pregnancy, from a study-specific delivery protocol, and from the obstetric record system. These data were analyzed using unadjusted and adjusted multinomial and logistic regression models.ResultsA total of 644 women were included in the study sample. Fetal weight exceeding 4000 g and vacuum extraction were found to be independent risk factors for both second-degree perineal tears (adjusted odds ratio 2.26 (95% CI: 1.22, 4.19) and 3.28 (95% CI: 1.73, 6.21) respectively) and OASI (adjusted odds ratio 5.93 (95% CI: 2.29, 15.33) and 4.31 (95% CI: 1.49, 12.5) respectively), whereas age above 25 years only increased the risk of second-degree perineal tears (adjusted odds ratio 1.94 (95% CI: 1.10, 3.43)). Heredity of pelvic floor dysfunction and/or connective tissue deficiency, induced labor, and fetal head circumference exceeding 35 cm were independent risk factors for high vaginal tears (adjusted odds ratio 2.33 (95% CI: 1.09, 5.00), 3.64 (95% CI: 1.54, 8.59) and 2.86 (95% CI: 1.43, 5.75) respectively).ConclusionsThis prospective study showed that fetal weight exceeding 4000 g and vacuum extraction are independent risk factors for both second-degree perineal tears and OASI in primiparous women. Risk factors for high vaginal tears were heredity of pelvic floor dysfunction and/or connective tissue deficiency, induced labor, and fetal head circumference > 35 cm. These tears have been found to be common among primiparous women, and merit more attention.Brief summary: This prospective cohort study showed that fetal birthweight exceeding 4000 g and vacuum extraction increased the risk of second-degree tear and anal sphincter injury.