The post partum period is characterized by a rapid shift in the hormonal environment. Within the first 48 hours after delivery, estrogen and progesterone concentrations fall dramatically. As these gonadal steroids modulate neurotransmitter systems involved in the regulation of mood, many investigators have proposed a role for these hormonal shirts in the emergence of postpartum affective illness.
While it appears that there is no consistent correlation between serum levels of estrogen, progesterone, cortisol, or thyroid hormones and the occurrence of postpartum mood disturbance, some investigators hypothesize that there is a subgroup of women who are particularly sensitive to the hormonal changes that take place after delivery. This population of women may be more vulnerable to PPD and to other hormonally driven mood disturbances, such as those occurring during the premenstrual phase of the menstrual cycle or during the perimenopause.
Other factors may play a role in the etiology of post partum depression. One of the most consistent findings is that among women who report marital dissatisfaction and/or inadequate social supports, post partum depressive illness is more common. Several investigators have also demonstrated that stressful life events occurring either during pregnancy or near the time of delivery appear to increase the likelihood of post partum depression.
While all of these factors may act together to cause PPD, the emergence of this disorder probably reflects an underlying vulnerability to affective illness. Women with histories of major depression or bipolar disorder are more vulnerable to PPD, and women who develop PPD will often go on to have recurrent episodes of depression unrelated to pregnancy or childbirth.