the girls are only there at night and are hypnotized to not remember the day, thus thinking themselves isolated with no outside contact, while during the day they live their usual superhero lives unaware of their night captivity.
So they probably take their own contraceptives during the day, especially because, as far as I know as a male, periods are an annoyance at best and really painful at worst, and make emotional control a pain, and contraceptives lower the worse effects. Something essential to superpowered crimefighters.