Michael Phelps is Dad: Fiancee Gives Birth to a Boy

— -- Michael Phelps is a first-time father.

The 18-time Olympic gold medalist and fiancee Nicole Johnson welcomed Boomer Robert Phelps on Thursday night in Arizona, where Phelps has been training in preparation for the Rio Olympics.

Phelps' longtime coach, Bob Bowman, confirmed that the baby's middle name is in his honor, calling it "a big surprise" in a text to The Associated Press.

Phelps had been training at high altitude in Colorado and left the Olympic Training Center in Colorado Springs to return to Johnson's side.

Last month at a meet in Mesa, Arizona, Phelps said the couple had already selected the baby's name, although he didn't reveal it then.

"It's something we both agreed on, thought it was different," he said.

Asked what type of parent he would be, Phelps said, "I'll probably be very protective. That's how I am."

Phelps credited Johnson for being "nothing but one of the best supporters I could ask for."

At the Mesa meet, Phelps talked with Dana Vollmer, who is attempting to make another Olympic team after giving birth to her son 13 months ago.

"It will be good for him, like it is for me," she said. "Michael and Nicole will be good parents."

Information from The Associated Press was used in this report.