Postpartum depression is thought to be triggered by the hormonal changes that follow childbirth. During pregnancy, the levels of the hormones, estrogen and progesterone rise greatly. Within about 24 hours of giving birth, however, the levels of these hormones quickly return to normal. Women who are prone to postpartum depression are thought to have brains which are unusually sensitive to these changes.
A postpartum drop in thyroid hormone levels may also factor into postpartum depression. Low levels of thyroid hormone may be associated with symptoms of depression. This condition is easily detected by a blood test and may be treated by taking supplemental thyroid hormone in the form of a pill.
It has also been hypothesized that a postpartum drop in insulin levels could play a role. Several studies have shown that insulin increases gradually during pregnancy, with secretion eventually doubling during the last trimester. Levels peak right before delivery and then quickly return to normal following the birth of the baby. Because some studies have shown that insulin affects the secretion of serotonin, which is believed to be involved in mood regulation, it is thought that the rapid drop in insulin following delivery might somehow affect serotonin release, causing depression.
Other factors which have been linked to postpartum depression include the stress and fatigue associated with adapting to a new baby, a lack of social support and a previous history of depression.
Sources:
Chen, T.H. "Postpartum mood disorders may be related to a decreased insulin level after delivery." Medical Hypotheses. 66.4 (2006): 820-3. Gabbe, Steven G. et. al. Obstetrics: Normal and Problem Pregnancies. 5th ed. Philadelphia: Churchill Livingstone, 2007. Jacobson, James L. and Alan M. Jacobson. Psychiatric Secrets. 2nd ed. Philadelphia: Hanley & Belfus, 2001.
