I have a bad back to begin with, so I started looking for a support belt at around 20 weeks. My belly wasn't really big yet, but working 12-hour shifts put enough extra strain on me that I already needed some help. I went to Babies R Us, and they had this belt in a S/M, so I tried it on, but it was too small. It seemed like it would be comfortable and help my back at least a little bit, so I ordered the M/L online. It's huge! I'm 29 weeks now, my belly is definitely bigger, and the belt still fits me like a hula hoop. I wear a size 12 in pants, and haven't gained much weight anywhere other than my belly, but neither size fits. If you are a true 'medium' size, you might be out of luck with this one - it seems the only options are small and large.