We do not know for sure. There are many anecdotal reports (personal accounts) of lupus flaring with pregnancy, the menstrual cycle, birth control pills, and hormone replacement therapy. We suspect that hormones play a role, but we don't know precisely what the role is. Lupus has a 9:1 female to male ratio so it is likely that hormones play a role, perhaps by influencing the immune system. Also, we know that female hormones have a definite effect on lupus mice used in research.