-- And then there were four.
"So excited!! Wonder if it's a boy or a girl??" Phelps, 32, wrote next to a picture of his son, Boomer, and wife, Nicole.
Nicole Phelps posted as well, writing, "Lil mans going to be a Big Brother!!!"
The 23-time gold medalist wed Nicole Phelps last year, and prior to that, they welcomed Boomer earlier that year.
Boomer, who has his own Instagram account, also chimed in on all the fun news.
"Do I get to have a brother or a sister?!?" he -- or someone controlling his account -- wrote.