The recently wed stars will welcome another baby into their family, which also includes Bronx, Ashlee's 6-year-old son. (His dad is Simpson's first husband, Pete Wentz.)
Simpson, 30, and Ross, 26, got married this past August in Greenwich, Conn. at the home of his mother, Motown legend Diana Ross.
Ross hinted at the possibility of growing his family with his wife last month at The Hunger Games: Mockingjay - Part 1 L.A. premiere. "I would love to [have kids]," he told Us. "As many as possible."
Actor and singer Ross added of married life: "It's the best thing ever. I actually want to get back to her, although I'm excited to be on the carpet — it's incredible."
The baby-to-be is the 3rd grandchild of Supremes singer Diana, 70.
Both Simpson and Ross' reps did not respond to requests for comment. E! News was first to report the story.