Evan, the son of Diana Ross, also shared the snap to Instagram with the caption: "The fam is growing. ♥️ Ash and I can’t wait to welcome the newest addition."
"Ashlee has been wanting another baby for a long time. They always wanted a big family," a source close to the 7th Heaven alum told PEOPLE.
She also told Hollywood Life: "We're working on it. Maybe there will be some news at some point. We still got a lot of things happening, but we definitely want to have another... we’re always practicing.”
The happy baby news comes after the spouses celebrated their three year wedding anniversary, with the "Pieces Of Me" singer posting a sweet tribute to her hubby on Instagram.
"Happy Anniversary my love 😍 it has been the best 3 years of my life ❤️," Ashlee captioned a throwback photo of the couple sharing a kiss on their wedding day.
"I love knowing that I get to wake up next to you for the rest of my life. Thanks for giving me the best love," she added.
The couple tied the knot in 2014 after they were introduced by her megastar mother-in-law, Diana.
Ashlee is also mum to 11-year-old son Bronx who she shares with her ex-boyfriend, Pete Wentz.