The couple, who welcomed their first child in August, were spotted taking their baby boy to the special ceremony at St Martin's Catholic Church in Brentwood, California, reported Us magazine.
"She looked beautiful and slim. Her mom was next to her looking on proudly. When they left, Axl was asleep in Josh's arms. They headed home for a small party to celebrate," a source said.
The Black Eyed Peas star, 38, wore a white ensemble, to match her son, while 'Safe Haven' star, 40-year-old Duhamel looked smart in a black suit and red tie.