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Any day now, Michael Phelps's 1-year-old son Boomer will become a big brother!
Phelps and his wife Nicole took to Instagram earlier this year to announce the exciting news.
But whether they are having a boy or a girl remains a big secret.
"We know the sex of the baby. We're not telling. That's, like, the one thing we're keeping to ourselves," Phelps told TODAY Parents.
"I'm voting boy. Nicole's voting girl. Obviously one of us is right," added Phelps who is a global ambassador for Colgate's "Save Water" campaign to promote everyday steps for water conservation.
As for the name, they're keeping that a secret, too, but they have a list of 10 names that they'e narrowed it down to.
Boomer and his new sibling will be about 22 months apart, and Phelps said it was important to the couple that their kids are close in age.
"I think they're close enough where it's gonna be perfect. I grew up with two older sisters, five and seven years older," said Phelps. "We wanted our first to have somebody close in age, so they could grow up together, and they could go through things together and they could play with each other."
For Phelps, Nicole's pregnancy is extra-special as he is able to be there every step of the way this time.
"I think I'm probably more excited this time around because I didn't go through the whole entire process with Boomer," he explained. "You know, from me traveling, and competing, and getting ready for Rio, I missed some months here and there leading up to the pregnancy and then the first couple months of his life when I was traveling at trials and the Olympics."
He's excited to be able to see the whole thing through, both the good and difficult times.
"This pregnancy I think is a little more challenging for Nicole. Me at times as well. But it's an experience. Nicole and I still are learning so much about each other every single day and it's been a fun process."
As Phelps and Nicole prepare for their second child, they also have to get one one-year-old Boomer ready, too.
"I think it was yesterday or two days ago he went over and felt Nicole's belly," he explained. "You know, the more Booms' personality comes out and he's doing sign language and communicating a little bit with us. He's just like a little human."
The Olympian continued, "He walks and kinda talks. It's been a cool experience. And having two, it's probably gonna be a big change and it will be challenging. But it's something Nicole and I are looking forward to."