Previous abortions and risk of pre-eclampsia

Background The risk of pre-eclampsia is reduced for second and later births. The causes and mechanisms behind this reduction are unknown. The aim of the study was to estimate the risk of pre-eclampsia in primiparous women according to history of spontaneous and induced abortions, while controlling for several potentially confounding factors. Methods The sample consisted of 20 846 primiparous women participating in the Norwegian Mother and Child Cohort Study (MoBa). Information on abortions and confounders were self-reported in postal questionnaires. The diagnosis of pre-eclampsia was retrieved from the Medical Birth Registry of Norway. Estimation and confounder control was performed with multiple, logistic regression. Results One previous induced abortion reduced the risk moderately [odds ratio (OR) 0.84, 95 confidence interval (CI) 0.691.02]. Two or more induced abortions reduced the risk more significantly (OR 0.36, 95 CI 0.180.73). Adjustment for confounders did not change the estimates. Conclusions The protective effect of two prior induced abortions was similar to what is commonly seen after one birth. Spontaneous abortions may to a larger extent than induced abortions be associated with other factors, such as infertility, that may increase the risk of pre-eclampsia. Normal pregnancies interrupted in early pregnancy may induce immunological changes that reduce the risk of pre-eclampsia in a subsequent pregnancy.