If you fall between sizes, go larger for a looser fit or smaller for a firmer fit. 
Whichever size you find best, this is also your correct size for the Mum Bums.

There are 3 height options for the Cheeky Bums; low rise, mid rise & high rise.
Low rise sits low under your belly button. Mid rise sits similar to the Mum Bums, just on the belly button. High rise sits up over the belly button.

There are 2 height options for the Super Cheeky Bums; mid rise & high rise.