Johnson Instagrammed the same photo, writing, "Lil mans going to be a Big Brother!!!"
Phelps and Johnson, both 32, have been married since last June, when they secretly tied the knot. Last October, he hinted at wanting another child during a Facebook Live video, in which he also confirmed their secret marriage.
"Just because," he said when asked about the decision to initially keep the wedding news quiet. "Gotta always have something, guys. Thank you for the beautiful comment. Baby number two may be coming soon. Who knows, though?"