The 39 year-old and actor husband Ben Affleck have named their new arrival Samuel, and are delighted with their son.
Jennifer and Ben announced they were expecting in August, and Jennifer told Jay Leno in January that they knew the sex of the baby, but weren't sharing.
However, according to US Weekly, Jennifer wanted a girl – while Ben said he'd be happy either way.
Jennifer told Jay Leno: "I would have thought [he wanted a boy]. At first...I really thought so...And then [Ben] kind of said, 'Well, we have girls. We know how to do girls. My girls love me. I'm the big guy in the house.' So, now I'm not sure."
She later told Ellen DeGeneres: "It would be so weird to have a boy," but also said the experience would be "cool and different."
Earlier this month the couple revealed they had decided to let their daughters choose the name of their unborn child.
"Our girls are working on names. At first they were definitely Disney. It was like, Donald Duck, Minnie Mouse Affleck. And then they've moved on. Then it was Peter Pan, Captain Hook, Smee," Jennifer told E! Online.
"They're coming up with lists and coming in and saying, 'Let's have a baby-naming contest! Let's have a baby-naming poll! They ask everyone, because they just want to know. But we're not telling."
Whatever the couple name their child, we've got our fingers crossed that it will be as classy as Violet and Seraphina.