The trick is when you know your Sim is pregnant, but before they have done the spin into maternity wear, you take them to the dresser and give them a new outfit in the designated maternity wear. There are very specific clothes that are used for pregnancy so you have to make sure you put them in those specific clothes. The most common are the empire dress nightgown, the tshirt over a long sleeved shirt, workout pants, most of the Generations EP clothes, high waist base game jeans and the zip up workout top. On the board that Sportsfan shared are some clothes that members have made specifically for maternity wear and they're really quite wonderful.
