I really haven't seen many below the knee skirts in maternity. It seems like maternity skirts in general tend to be on the casual side. Can you go with dresses instead? There are so many great maternity dresses!
Oh, and I hear you about needing maternity clothes earlier than expected. I pulled out a pair of pants this morning and I could swear I wore them last week. There was no chance the zipper was going up. 
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