While the timing of symptoms suggests hormonal fluctuations as the cause of PMDD, a demonstrable hormonal imbalance in women with PMDD has not been identified. In fact, levels of reproductive hormones and their metabolites in women with and without PMDD are indistinguishable. It is instead hypothesized that women with PMDD are more sensitive to normal levels of hormone fluctuations, predominantly estrogen and progesterone, which produces biochemical events in the nervous system that cause the premenstrual symptoms. These symptoms are more predominant in women who have a predisposition to the disorder.