I have a pair of black maternity trousers that shrank in the wash, and they were petites to start with.
they are size 16-18 too, but they were £50 and I cannot throw them out, I only wore them once!. Free to anyone who will fit in them - I know the trouble I had getting short enough maternity trousers, I hope I can give someone else a chance to wear trousers that fit!