Evidence suggests that hormonal changes may play a role. Understanding the neuroendocrinology characteristic of PPD has proven to be particularly challenging given the erratic changes to the brain and biological systems during pregnancy and postpartum. A review of exploratory studies in PPD have observed that women with PPD, have more dramatic changes in HPA axis activity, however directionality of specific hormone increases or decreases remain mixed. Hormones which have been studied include estrogen, progesterone, thyroid hormone, testosterone, corticotropin releasing hormone, endorphins, and cortisol. Estrogen and progesterone levels drop back to pre-pregnancy levels within 24 hours of giving birth, and that sudden change may cause it. Aberrant steroid hormone–dependent regulation of neuronal calcium influx via extracellular matrix proteins and membrane receptors involved in responding to the cell’s microenvironment might be important in conferring biological risk. The use of synthetic oxytocin, a birth-inducing drug, has been linked to increased rates of postpartum depression and anxiety.