US swimming star Michael Phelps, the most decorated Olympian of all time, took to Instagram, on Wednesday, to share the news that he and fiancée Nicole Johnson, a former Miss California USA, are expecting their first child.

The social networking site had an adorable photo of the proud parents-to-be holding tiny trunks with the initials MP on it.

"We found out 8 weeks ago we were expecting," the swimming champion wrote beside the caption.

"Today we're over 12 weeks and it looks like we're having a boy,” he added.

Earlier this year, Phelps announced their engagement on social networking site.

"She said yes!!!!" Phelps, 29, posted on his Facebook page, along with a picture of him and Johnson playing in the snow.

This will be the first baby for the champion and Miss California USA 2010, both 30.