The singer famously penned All of Me for his then-fiancee in 2013, and now John has been inspired by impending fatherhood to create some new tunes for their baby.
However, Chrissy reveals John previously insisted he wasn't going to be "that guy" and gush about his children in song.
"A few years ago, a lot of people were writing songs when they got pregnant and he was like, 'I just feel like I'm not gonna be that guy, I'm not gonna write a song,'" she recalls. "(He said), 'I think Stevie (Wonder), with Isn't She Lovely?, how can you do anything better than that? I'm just not gonna be that guy.' And then yesterday he texted me, 'Writing a song for our baby!'."
Chrissy shared the sweet song news during Monday's (26Oct15) episode of her U.S. talk show FABLife, and admitted the couple was "really excited" to start its own family.
The mother-to-be, 29, had previously opened up on the programme about her struggle to conceive and the pair's attempts at in vitro fertilisation (IVF) treatment, which retired supermodel and co-host Tyra Banks also admitted to undergoing.
"I will say, honestly, John and I are having trouble," Chrissy said last month (Sep15). "We would have (had) kids five, six years ago if it'd happened. But my gosh, it's been a process!"
Weeks later, the couple announced its pregnancy online, alongside a snap of John sweetly touching his wife's stomach.
"John and I are so happy to announce that we are pregnant," the model wrote. "As many of you know, we've been trying to have a baby for a while now. It hasn't been easy, but we kept trying because we can't wait to bring our first child into the world and grow our family.
"We're so excited that it's finally happening. Thank you for all your love and well wishes. I look forward to all the belly touching!"
John and Chrissy began dating in 2007 and wed in 2013.