Is depression more likely following childbirth? A population-based study.

Postpartum depression (PPD) is characterized as a depressive episode conditional on childbirth. We examined whether the risk of depression is higher following childbirth than that at a randomly generated time unrelated to childbirth. In a prospective cohort of all women with live singleton births in Sweden, 1997-2008, we first calculated the relative risk (RR) of PPD for mothers with a history of depression compared to mothers without such a history. Next, we repeated the calculations, but now for depression following a computer-generated arbitrary "phantom delivery" date, unrelated to the true date of delivery. For this phantom delivery date, we used the average expected date of delivery for all women of the same age. For the analyses of each group, women were followed for a full calendar year. We fitted Poisson regression and calculated RR and two-sided 95% confidence intervals (CI). Among a total of 707,701 deliveries, there were 4397 PPD cases and 4687 control depression cases. The RR of PPD was 21.0 (CI 19.7-22.4). The RR of depression in the control group was 26.2 (CI 24.7-27.9). We provide evidence that the risk for PPD is no greater following childbirth than following a random date unrelated to childbirth. This finding suggests that the postpartum period may not necessarily represent a time of heightened vulnerability for clinically significant depression and that the well-established observation of depression covarying with childbirth does not necessarily equate to causation, but rather may be a secondary effect of postpartum women representing a medically captured population.