Exactly what Christy said!! Althoug for the stuff I bought online I bought the next size up from what I would normally buy (I was a 12 so bought 14). I haven't had so buy anything from 20wks.

Though I guess it would depend on how much weight you put on....I remember I was horrifed at one store saying I owuld need XXL - but bought them and held them up with a safety pin - just incase you know? - now they fit great!