Puerperal psychosis is a complex and rare disorder (1 out of 1000 births), which may be the first episode of a mood disorder such as bipolar disorder. The post-partum time is a key period, regarding social, affective, psychological, biological and hormonal status. Being primiparous, single and having a Caesarean are the only non-clinical known risk factors. Genetic factors may be involved in the risk for post-partum psychosis according to numerous familial studies. Molecular genetic studies even showed a significant role of the “s” (short) allele of the SERT (serotonin transporter gene). It is proposed that post-partum psychosis represents a highlighting example of the “genetic vulnerability” concept, as some patients at lower risk may express their (lower) vulnerability only when meeting severe triggering conditions such as delivery.