"Two under two is tough," Reynolds said. "A lot of estrogen. It's like an American Girl doll took a [expletive] on my life."
The baby, whose name has not yet been revealed, joins big sister James, who turns two in January.
Reynolds also joked with host Conan O'Brien about the reasons for his appearance on the show, which included promoting a new line of ham-based underwear to a fictional parenting book called 10 Easy Steps to Lower Your Child's Self-Esteem.
"Who wants a cocky, know-it-all kid? Not this guy," he said.
Eventually he revealed he was there to promote The Notebook 2, which also starred "Conan McAdams."
"I don't recall being involved in anything like this," O'Brien said.
"I'm never going home again," Reynolds said following the clip.
Lively later shared a photo to her Instagram showing Reynolds' attempt to make up for his kiss with O'Brien.
"The ways I know that @vancityreynolds got himself into huge trouble on @teamcoco tonight," Lively wrote. "Not only did he come home with my true love, @levain_bakery cookies, but he personally toasted them for me and brought me a cup of milk WITH ICE. Literal brownie points."
"#IfYoureABirdImABird," she quoted from The Notebook.