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) 0.69-1.02]. Two or more induced abortions reduced the risk more significantly (OR 0.36, 95% CI 0.18-0.73). 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.