The couple, who welcomed their first child, Axl, into the world in August, were spotted taking their baby boy to the special ceremony at St. Martin's Catholic Church in Brentwood, California, where they were joined by approximately 10 family members and close friends.
An eyewitness told UsMagazine.com: ''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.''
The Black Eyed Peas star, 38, wore a white ensemble, to match her son, while 'Safe Haven' star, Josh, 40, looked smart in a black suit and red tie.
Fergie recently gushed about her love of parenthood.
She said: ''Baby Axl is amazing. He's the light of my life.''
The blonde beauty initially struggled to adapt to her son's unpredictable routine but has since learnt to ''live more spontaneously.''
She explained: ''I'm a scheduler, I'm a planner, so I would schedule the time to spend with him for learning time. What I didn't realise is his feeding times change every day. So I just really have to live more spontaneously and just, kind of, when he's ready to learn and be fed and he's ready to react and respond to me - he's first priority - and my emails will have to wait.''
Josh previously admitted he is already eager for them to have another child.
He said: ''We've gotta move fast! [Axl's] a really good little baby, which means the second one will be a holy terror!''
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