“I wrote it for my wife,” Kelly said as she performed the song “Twin Flames” at the 2022 Billboard Music Awards on Sunday.
The ceremony took place the day before Fox’s birthday, and Kelly said “Happy Birthday my love” during the performance.
He added: “This is for our unborn child.”
Neither has announced marriage or pregnancy, and Kelly has previously referred to Fox as his “wife.”
He has a daughter, Kathy, 12, from a previous relationship, while Fox shares three sons with her ex-husband, actor Brian Austin Green, Noah, 9, Bodhi, 8, and Journey, 5.
CNN has reached out to Kelly’s representatives for comment.