So you can take it off one handed after the birth, but aren't worried about it coming loose in the course of events, I'd make a wrap style skirt.
1 and 1/2 times your waist measurement for the width
waist to where you want the skirt to end + 1" for the height
You'll use the 1" on the height to finish your edges. Personally, I would use a knit fabric and not bother with finishing the edges.
For the easiest version, to position your fastening method (hook and loop tape (e.g. Velcro) would be nice and smooth and secure) just wrap your fabric comfortably around your waist and mark where the ends of the fabric land on the middle part of the fabric.
Or if you want a more traditional wrapped sarong, get a piece of fabric about the same size as a beach towel and just
play around until you feel comfy. That method might not be as effective at allowing free movement, however.