Go for it!!!When I bought my Medela PIS advanced it came with the medium size breast shield. Using that size caused a ring around my breast and soreness. Pumping was very painful and milk would leak out of the sides of the shield.So I took a chance and went down a size to the 21 mm shield. It was an great investment! I get more milk now and it doesn't hurt at all. A lactation consultant over the phone told me most women need to go up a size and I probably needed to get the large size too. She was way wrong. The smaller size fits better.If you aren't fitting correctly then try this size.