Familial risk of postpartum psychosis

BACKGROUND: Postpartum psychosis, a mood disorder triggered by childbirth, is one of the most severe psychiatric conditions, with high risks of suicide and infanticide if untreated. While it is evident that genetic factors play a crucial role in disorder risk, the exact extent of their importance is yet to be determined. METHODS: This cohort study consisted of 1,633,535 birthing parents from the Swedish nationwide registers, of whom 2,489 (0·15%) experienced postpartum psychosis within three months of their first ever childbirth. We estimated the relative recurrence risk of postpartum psychosis for full siblings and cousins as a measure of familial, genetic, and environmental risk. FINDINGS: Relative recurrence risk of postpartum psychosis in full siblings was 13·77 (95% CI 8·52–20·91) when adjusted for age at birth. Although cousins showed an elevated relative recurrence risk, these results did not reach statistical significance (1·88 [95% CI 0·74–3·82]). In full siblings, the relative recurrence risk for severe postpartum psychosis requiring inpatient diagnosis was even higher than the risk for inpatient and outpatient diagnoses (18·13 [95% CI 11·12–27·57]). INTERPRETATION: Increased risk of postpartum psychosis in full siblings is likely due to a combination of genetic factors and shared environment. The elevated risk in cousin pairs, lower than in full siblings, highlights genetic impact because shared environmental effects are assumed to be minimal among cousins. However, caution is needed in interpreting the risk among cousins due to wide confidence intervals. Overall, our study supports the role of genetics and shared environment in the risk of postpartum psychosis.