3-d period

When a period is so heavy that it's thick and chunky

Boy: "Hey wanna have period sex?"
Girl: "We really shouldn't, this is a 3-D period"

2-D period

When a period is light enough that it only produces a very liquified waste; the viscosity of a 2-D period is extremely thin

Boy: "Hey how's your period?"
Girl: "Light enough that we can have sex, it's only a 2-D period"