The funnyman made the comment at his show last night at London's Hackney Empire.
Brand was telling the audience about a former fling and new mum who used to let him drink her breast milk.
He quipped that his soon-to-be missus Perry 'doesn't mind'.
'She likes it which bodes well for the first pregnancy,' he added.
Brand also joked that he'd ditch his future wife for her pop rival Lady Gaga.
The comedian told the audience that he liked that Perry was 'sexy' and 'desirable to others'.
'Frankly, if she lost them I'd be on the phone to Lady Gaga,' he said.
The comment is sure to go down like a lead balloon with his soon-to-be missus, who has rubbished Gaga on Twitter in the past.
Perry took to the social networking site to slam Gaga's raunchy video for Alejandro, which saw the kooky singer involved in some intense bondage manoeuvres and gobbling rosary beads while dressed as a nun.
She tweeted: ‘Using blasphemy as entertainment is as cheap as a comedian telling a fart joke.’
Brand's show was part of a promotional tour for My Booky Wook 2: This Time It's Personal - the follow-up to the comedian's first volume of memoirs.
In the first half, he read extracts from his new book before being interviewed on stage by old pal Jonathan Ross.
