Usually initiated by the guy, an act and symbol of relief that one's lady friend is menstruating and thus, not pregnant. For example, if your girlfriend is on an oral contraceptive and you decide to pull the goalie for the season, one may start to worry that the pill may not be 100% effective. In this instance, one is excited when that time of the month comes and upon hearing of Aunt Flow's visit, a menstrual high-five is warranted.
Guy: hey babe, this party blows. Wanna sneak into the laundry room and fuck on their dryer?
Girl: I can't. I got my period this morning...
Guy: Whew! Menstrual high-five!!!
*puts up arm with a Todd-from-Scrubs-like grin*