The condition affects around one in 500 new mothers, and includes mood disorders.
It is believed that the mood disorders were caused by women's circumstances, personality and hormonal changes.
The scientists have started their work to isolate the gene that would enable doctors to identify and treat women at risk of postpartum psychosis, before they are actually affected.
"Postpartum psychosis is classed as among the most severe episodes of illness seen in clinical practice," the Telegraph quoted Dr. Ian Jones, head of psychological medicine at Cardiff, as saying.
The researchers are studying the DNA of families in which at least one woman had experienced postpartum psychosis.
"We have identified chromosomal regions that are likely to harbour genes that predispose individuals to (postpartum psychosis)," he said.
"The consequences for the mother, infant and family are so serious that such episodes require close attention, often including hospitalisation," he added.