Why not add in a pair of coordinating lounging pants? 

Made of cozy flannel in solid colors that will compliment the main fabric chosen, your lounging pants will also help to keep you warm.  And of course, adding in some pants will help shed any lingering ideas about this being a dress!  While they most likely can't be worn into the actual operating room, these pants will add extra comfort and assurance of no flashing both pre and post op.