Doku played 86 minutes of Belgium’s opening 1-1 draw with Egypt in Group G but missed their 0-0 draw against Iran.
His absence was explained as illness, with wife Shireen due to give birth during the second week of July.
However, Doku had actually travelled back to London for the birth of his son Praise.
A Belgium statement on Monday said, external: “With the approval of and accompanied by one of our team doctors, Jeremy travelled to London to be with his wife for this unique occasion.
“Jeremy will rejoin the squad on Tuesday evening in Seattle as preparations continue for the next match against New Zealand.”
Before heading back to Europe the winger was adamant of the importance of being at the birth.
“If you ask me what I want, my answer is that nobody wants to miss the birth of their first child,” Doku, 24, told Reuters.
“But I also know that football involves many other considerations. I know the federation supports its players and understands their situations. We’ll see what we can do.”
England striker Ollie Watkins, who has two children, backed Doku.
“I think someone labelled it disgusting and I think for a start that’s not a way to label a birth,” he said. “I’ve seen what my wife had to go through and that was quite smooth sailing but I know family members and friends that haven’t had it that way.
“It only happens once – welcoming your first child to the world – and it is a blessing. There’s a lot of times where you’re away from family and friends during the season and it’s very difficult, so to miss that would be tough and I see where he’s coming from.”
The Professional Footballers’ Association said demands placed on players should not be at the expense of “fundamental family moments”.
“While every situation is different, we believe players should be supported in balancing their professional responsibilities with important life events,” a PFA spokesperson said.
“Supporting players as people, not just athletes, is an important part of creating a healthy professional working environment.”
The Fatherhood Institute, which supports men as hands-on fathers and caregivers, also backed Doku.
“It makes me think of gladiators in the Colosseum,” deputy chief executive Jeremy Davies told BBC Sport.
“We want these men to be these heroic figures who exist for our entertainment. They get paid lots of money but there are some things that are worth a lot more.”