Proud new mum Coleen Rooney was behind the choice of name for her baby with husband Wayne, it has been revealed.
The footballer's grandmother Pat Morrey revealed it was Coleen who picked out Kai, though she didn't know the reasons behind the choice.
The name has several different meanings around the world. It has strong Scandinavian origins, and means 'rejoice' in Finnish, while in Japanese and Hawaiian it signifies the 'ocean'.
In keeping with the Rooney family tradition, Kai has been given the middle name Wayne - also the name of the sport star's dad.
Twenty-four-year-old Wayne was on hand for the new arrival. He didn't leave his wife's side from the moment they arrived at the hospital, just after 10pm on Sunday night.
It is believed medics decided to induce the baby as he was nine days overdue. He was expected to share his dad's October 24 birthday, but instead arrived at 2.20pm on Monday, during a natural birth.
A source close to the couple said that when Wayne was handed his son for the first time he welled up and said: "I can't believe it."
The soccer ace is expected to be excused from his team United's Champions League clash with CSKA Moscow tonight to be with his wife and child.
Coleen, who eschewed private healthcare to give birth at the NHS Liverpool Women's Hospital, is expected to return home on Tuesday evening.