I wear a size XS Pea in The Pod/ Motherhood maternity clothes. I ordered this in a small and it was great for a Pregnancy costume. No matter how big your belly is just order the size you would normally be. The skirt portion will just be up high like a maternity shirt. I wore black pants underneath. The costume was the best! Only complaint is that the plastic material was smelly & hot if you were not out in the cool air. I aired the costume out and it was great!