Carson is awaiting the arrival of her baby boy, while Arnold recently gave birth to a girl.
In the sweet shot, the DWTS dancers are all smiles in front of a balloon archway and a light-up sign. Carson, wearing a midi dress, cradles her bump in the pic, while Arnold, wearing jeans and a striped cardigan, grins next to her pal.
"Loved celebrating this beautiful mama to be ❤️❤️," Arnold captioned the pic. "I am so happy for you @witneycarson cannot wait to meet baby boy!!"
In an Instagram Story following the shower, Carson said she was "grateful" for the event, which she called "so magical... so special, so beautiful."
Arnold's post came about two weeks after she welcomed her first child, a baby girl named Sage, with her husband, Samuel Cusick. ET spoke with Carson and Arnold when they were both pregnant, and the DWTS pros gushed about sharing the experience with each other.
"It's actually insane. We've done so many things in our lives together, so it's like, naturally, why wouldn't we get pregnant together?" Arnold joked. "It's so funny because you can't really plan a pregnancy. I mean, you can plan when you want to try, but it's not like it's always going to happen when you want it to."
"Now we're just comparing our bellies," Carson added. "Like, can you believe we both have bellies right now? It's really weird."