Background. Conflicting evidence exists on causes of vulnerability to post-natal depression. We investigated genetic and environmental influences on variation in post-natal depressive symptoms (PNDS) following first live birth, and sources of covariation with the personality trait Neuroticism and lifetime major depression occurring post-natally (DEP-PN) and at other times (DEP-XPN) to test for shared genetic influences.

Method. Retrospective interview and questionnaire data from 838 parous female twin pairs (539 monozygotic, 299 dizygotic) from the Australian National Health and Medical Research Council volunteer adult twin register were used for multivariate genetic model-fitting. Data on PNDS were evaluated for consistency with diagnostic interview assessment.

Results. Genetic factors explained 38% of variance in PNDS (95% confidence interval 26–49%) and 25% of the variance in interview-assessed DEP-PN. The genetic correlation between PNDS and lifetime major depression (DEP-PN and DEP-XPN) was low (rg = 0·17, 95% confidence interval = 0·09–0·28), suggesting that the questionnaire was measuring a construct other than post-natally occurring major depression, possibly post-natal dysphoria. Associations between PNDS and obstetric factors were very modest.

Conclusions. Findings suggest modest genetic influences on major depression occurring post-natally. Independent and stronger genetic influences identified for post-natal symptomatology or dysphoria (PNDS) justify further investigation.