Hormones

During pregnancy the levels of the female hormone estrogen and progesterone increase up to ten fold. After delivery these hormone levels decline rapidly, reaching pre-pregnancy levels in about 72 hours. Some researchers have demonstrated a link between rapidly declining hormone levels and the appearance of symptoms associated with depression in the post partum period.
More recent research is focusing on the relationship between prolactin levels and symptoms of depression. Prolactin levels are lower at the time of delivery but begin to rise sharply in the first week following delivery.

Changes in the blood levels of adrenal steroids like cortisol and aldosterone occur in the post partum period and researchers believe there could be a link between these changes and the frequency with which women experience mood changes in the post partum period.
Some of the current research indicates that a relationship exists between premenstrual syndrome (PMS) and post partum depression, with higher rates of depression being experienced by women who suffer from PMS