Its no differeny than regular clothes. If you were a small get a small , large a large. True, some maternity clothes runs big but read reviews on things purchased online.
Im 36wks and never bought maternity clothing. I just wear strech pants and sweats. I was a small in pants so I just bought mediums. And a size bigger in shirts. :)
I went to gordmans to try on a few things, then I ordered some things from cheaper sites like Walmart.com etc. a lot of maternity clothes run larger than normal sizes.