Michael Phelps and fiancée Nicole reveal they are expecting their first child!

The most decorated Olympic sport star of all time has revealed to the world via Instagram that he is expecting his first child, and he already knows the gender!

American swimmer Michael Phelps, who is a 22-time medalist for team USA, shared a photo with the caption; 'We found out eight weeks ago that we were expecting.'

Using the hashtag 'life about to change' Michael, who is now tee-total after a public battle with alcohol, added; 'Today we're over 12 weeks and it looks like we're having a boy!'

On Wednesday the 30 year old posted the candid family snap with fiancée Nicole Johnson, 29, which features the proud sportsman hugging his bride-to-be's stomach.

In a nod to the 18-gold-medallist's swimming career, the reveal took place at a pool in Arizona.

[Instagram]https://instagram.com/p/-O21mISxyZ/?taken-by=m_phelps00[/Instagram]

Nicole can also be seen holding an adorable mini pair of Speedo swimming trunks emblazoned with the initials 'MP'.

In less than 24 hours the post has already received nearly 50,000 likes and 2,000 comments congratulating the pair on their news.

Nicole said; 'We couldn't be happier!'

Michael, who was born in Baltimore, Maryland, has a history of sharing the big milestones in their life via his social media page.

When they got engaged on 21st February this year, the couple, who have dated on and off since 2011, both posted snaps to their respective pages, with Michael captioning his; 'She said yes.'

[Instagram]https://instagram.com/p/zZOMeCn481/?taken-by=nicole.m.johnson[/instagram]

Whilst Nicole showed off her sparkling ring telling her followers; 'I'm gonna be a Mrs. Phelps.'

The star, who is reportedly worth $55 million, plans to marry the former Miss California next summer after the Olympic games in Brazil.

Based on the timings of the announcement the mini-me should arrive just in time to cheer on his famous father at the sporting event in Rio.

Better keep hold of those Speedos!