The couple, who already have 22-month-old son Axl Jack together, are believed to be considering going down the adoption route for their second child, reported OK! magazine.
"They think it's the right time to expand their family," a source said.
Meanwhile, an insider previously claimed that 40-year-old Fergie's "obsession" with the gym was halting any plans for her and Duhamel to have a baby.
Even Duhamel has made no secret of the fact he wants a brother or sister for little Axl.
"We've got to be very strategic in our planning for it. But the plan was to always have at least two," he said.