The Sims 3 Tips

Maternity clothes are customizeable
before your sim is showing have you sim go to the dresser and plan outfit and the "ugly" outfits are in there (the dress-like one is under full outfit well the other one is just is under "top" the other ones like formal, sleepwear are under the other categories) when you find the outfit you (your sim) can wear (dress-like outfit) customize it and it will work.