While there is no one designated cause for PPD, there are a few factors which can contribute to the onset of major depressive disorder after giving birth.

Similar to the end of the luteal phase in the menstrual cycle, childbirth can lead to a major drop off in serotonin and dopamine producers, estrogen and progesterone. This creates a major deficit in the "happier" sides of emotional regulation and can be part of the hormonal relation to PPD.

Additionally, many different sources have noticed a correlation between those who have low social support and/or high stress during pregnancy and the development of depressive symptoms after childbirth.