The couple’s new arrival joins their daughter Maxwell Drew, 6, and son Ace Knute, 5.
“Several of her closest girlfriends also just had babies and she’s thrilled to join the club and raise all of them together,” the source said. “This was such an amazing surprise and shock but truly feels right and meant to be for the family.”
The source added at the time, “She would love to be done with being pregnant but also knows its going to go fast and there’s a lot to do to get ready before the baby comes.”
Simpson had revealed on social media in September that she was pregnant with her third child and also revealed she was expecting another daughter.
“SURPRISE…” the fashion mogul wrote. “This little baby girl will make us a family of five. We couldn’t be happier to announce this precious blessing of life.”
She also shared photos of her and Johnson’s eldest kids at the family’s sex reveal party, holding up jumbo-sized balloons containing smaller pink ones. Simpson also posted a snap of herself cradling her baby bump.