Olympic champion swimmer Michael Phelps and fiancée hail birth of their first child and give him an unusual name

Michael Phelps, the 18-time Olympic champion swimmer and his fiancée, Nicole Johnson, are celebrating the birth of their first child, a baby boy called Boomer.

They announced the arrival on Instagram, with the baby being cuddled by his father as his mother looks on.

"Welcome Boomer Robert Phelps into the world!!! Born 5-5-2016 at 7:21 pm !!! Healthy and happy!!! Best feeling I have ever felt in my life!!! @nicole.m.johnson and Boomer both healthy!!!" he wrote.

Miss Johnson, the 2010 Miss California USA, has been going out with Phelps for more than six years, with the couple announcing their engagement last June.

It came after Phelps appeared to be winning his battle with alcohol, having gone into rehab after being arrested for drunk-driving for the second time.

He retired from the sport following the London 2012 Olympics, but returned to the sport 20 months later – but was banned by USA Swimming for failing a breath test.

He has  voiced hopes about competing in his fifth Olympics in Rio.

However in a recent interview with Associated Press, Phelps appeared optimistic about the future.

“I feel like I am okay and happy with who I am. I feel like I’ve learned so much about myself, who I truly am. I can honestly say there aren’t many people who have seen who I really am.”