I am 5'6, 6 months pregnant and I'm in a bra size 38F so I ordered these in an L.  I wasn't expecting them to be big on me, but they are.  I washed and dried them, and that helped a little, but they are still not as snug as I'd like them to be.  I'm on strict bed rest and just need something to lay around in, since these are so cheap I'll hang on to them.  I plan on ordering more, but I'll go with the M, hopefully the smaller bang/cups will offer more support.  I love the fact that these fit like a sports bra but without the racerback, which seems to give me a neck/headache.  These are basically like wearing a tight cutoff tanktop.  Perfect for sleeping (or bed rest).  These will also keep nursing pads in place nicely.