You have to go to a shop and then buy some more clothes off the clothing racks.
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The same way you changed clothes in the base game, and the same way you change clothes throughout the entire game: buy a dresser, click it, and choose one of the options to "Change into..." Sims can go to community lots to purchase additional clothing, at which point they can also have the option to plan their outfits, allowing the player to select which outfit will be changed into when the Sim switches between clothing categories.
go to a community lot that has clothing racks, then select the rack and pick buy clothing. it will bring up a window with all the clothing you can buy. once you have everything you wish to buy hit the check and your sim will purchase the clothing. when you get home, make sure you have a dresser so you can "Plan outfit" and change your sims' clothes.
You can get new maternity clothes for Sims 2 by downloading Squinges mod; pregnant Sims wear any outfit.
The Sims 2 bodyshop is a place where you can make custom sims and clothes for them.
Actually yes there is other infant clothes sims 2 PC try modthesims.com & thesimsresources.com and type infant or infant clothes kbye :)