You downloaded some maternity clothes and there so cute and so much better then the game maternity clothes but you don't know how to make your Sims wear them when pregnant, well I'm here to help you...
Before you get your Sim pregnant or right after before she is showing go to your dresser to change clothes.
once you get to CAS to change your Sims clothes find the maternity clothes you have downloaded choose it for your everyday clothes and ect. set up 3 for each set if you want to change her more then once.
Once you have set up your maternity clothes and got your Sim Pregnant
your Sim should still be able to ware the maternity clothes you have chosen for it.