The star's actress wife Courtney Ford is pregnant and due to give birth to the couple's first child this summer (12).
Routh admits he's delighted to finally be able to share the happy news - because he hated keeping it a secret from the cast and crew of his new TV pilot Partners.
He tells People.com, "It's been so hard keeping the news to ourselves. Everyone on set keeps asking, 'Do you have kids?' and I've had to say, 'Well... not yet!' It's the truth. The baby hasn't been born yet!"
And Ford has been struggling to hide her growing baby bump from her colleagues on the set of TV comedy Parenthood.
She adds, "It started getting difficult to hide what was happening. Flu and food poisoning excuses only work for so long, and by the last episode I could no longer button my pants! I just tried to joke that I had a big lunch!"
The couple wed in 2007.