“Number 2 on the way!!!! So excited!! Wonder if it’s a boy or a girl??” Phelps, 32, wrote alongside a photo of the couple’s adorable son Boomer clutching a pregnancy test while snuggled up in his mom’s arms.
Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.
Added Nicole in her own Instagram post featuring the photo, “Lil mans going to be a Big Brother!!!”
It seems Boomer, 15 months, is looking forward to his impending big brotherhood as well, with an Instagram post on his own account saying, “So excited to see what mama is having!! Do I get to have a brother or a sister?!?”
Phelps and wife Nicole welcomed Boomer in May 2016. They were wed later that year – twice! First the pair tied the knot in June and then again in late October during a larger event in Cabo San Lucas, Mexico.
Earlier this year, Phelps revealed to PEOPLE that his wife was looking forward to growing their family, sharing, “Nicole has recently brought up that she would like a girl!”
“We definitely want more kids, it just depends on when,” he added, noting that “I’m sure I’m getting pressure.” Though Phelps said Nicole’s pregnancy with Boomer wasn’t planned, this time around he teased, “[We’re] trying to get moving on some things!”
In another interview, the athlete further told PEOPLE of more children, “We are so fortunate to have a healthy and happy baby and hopefully more coming soon, but we don’t know when.”
“We’re going through life everyday happy and blessed with what we have. But I hope to have more kids in the future,” he said.
RELATED VIDEO: Michael Phelps Celebrates His First Day Of Retirement!
Now in retirement, the 23-Olympic-gold-medal holder has more time to focus on fatherhood. He told PEOPLE in July that he’s totally embraced every moment of watching Boomer learn and change with age, gushing, “I think that is the coolest thing, watching him grow, and watching him figure out new things.”