Get something thats not too tight but firm, you'll want something to help hold up those hospital "70's malibu surfboard" sized pads
Haha, those pads are so huge that ANY kind of knickers you wear, those pads will stick out a few feet back and front of your undies

Myself, I do belive I brought my biggest pair of underpants to the hospital, just for that reason. Wasn't there long though to use the surfboards ... erm ... 15 hours or so I think ... home is so much nicer.