Baby brouhaha!

A source tells Us Weekly that January Jones -- who revealed she was pregnant April 28 -- was so intent on starting a family that it drove a wedge between her and ex-beau Jason Sudeikis.

"She told him she wanted kids and it turned him off," says the insider of the actress, 33, and the Saturday Night Live star, 35. "Jason was bummed out she was on the mommy track." 

While Jones has yet to reveal the father of her baby, the source doubts it's Sudeikis: "If Jason were the dad, he'd step it up."

Mad Men producers have reportedly yet to decide whether Jones's pregnancy will be written into the show.