Evidence points to low levels of omega-3 fatty acid, chiefly found in oily fish, playing a key role.
A combination of omega-3 deficiency and genetic factors may trigger depression after childbirth, scientists believe.
Between 10% and 15% of mothers are believed to suffer post-natal, or post-partum depression (PPD) during the first year after birth. The condition is more serious than "baby blues", a mild reaction common within the first few days of delivery, but much less severe than post-partum psychosis which affects fewer than one in 500.
Mothers with PPD tend to be disengaged, unresponsive and even hostile towards their children.
The scientists analysed data from 75 previous studies on PPD looking at links with a genetic variant called 5-HTT and omega-3 intake.
Lead researcher Dr Gabriel Shapiro, from the University of Montreal in Canada, said: "There could be a link between pregnancy, omega-3 and the chemical reaction that enables serotonin, a mood regulator, to be released into our brains. Many women could bring their omega-3 intake to recommended levels."
Scientists said the evidence needed confirming by further research.
A mother's omega-3 levels fall during pregnancy and remain lowered for at least six weeks after birth, said the researchers.