Sometimes a woman in a shaky marriage hopes the child will cement her bond with her husband and becomes depressed when that expectation is disappointed. For women who already tend to feel inadequate and incompetent, difficulties in caring for the child may confirm their poor opinion of themselves. The mood of some women is disturbed by the change in their relationship with their husbands and the start of a new relationship with the baby. The situation becomes worse if they lack intimate friends to share their secrets and troubles. The universal expectation that a new mother should be happy further contributes to guilt and depression.
Hormones also play a role.
High levels of female sex hormones circulate in expectant mothers' blood, but drop precipitously within hours of delivery, contributing to biochemical depression. Pregnancy also increases levels of endorphins, the body's feel-good chemicals. Endorphin levels fall abruptly after delivery, adding to risk of depression. A history of depression increases risk. Women with postpartum depression are more likely to have had previous mental illnesses, depression during pregnancy, and a family history of mood disorders, especially bipolar disorder.