The couple are also parents to son Ace Knute, 5, and daughter Maxwell “Maxi” Drew, 6½.
The couple first revealed their happy news in August, with a little help from their kids.
“SURPRISE … ” Simpson, 38, captioned an image of the couple’s children, as they prepared to pop two large polka-dot balloons to find out if they would be welcoming a little brother or sister into their family.
“This little baby girl will make us a family of five,” she added alongside a snap of her kids surrounded by smaller pink balloons. “We couldn’t be happier to announce this precious blessing of life.”
In honor of her daughter on the way, Simpson threw a “Birdie’s Nest” baby shower in January, revealing her baby girl’s name in the process, that was complete with a neon sign and whimsical table settings encouraging guests to “stuff your beaks.”
“It was a great day. Jessica is feeling very loved and she is ready for [Birdie] to come,” a source revealed to PEOPLE at the time.
“She felt very loved, surrounded by her closest friends and her mom and her sister.”
“The shower was a bird’s nest theme to celebrate Birdie,” the source added, explaining that “Birdie is a family name” on the singer’s side.