Katy Perry used to "wish for twins" before having daughter Daisy.

The Firework singer welcomed baby Daisy Dove Bloom, her first child with fiance Orlando Bloom, into the world in August, and got back to work just weeks later filming TV show American Idol.

The latest episode of the talent show aired on Sunday night, with teenage siblings Liahona and Ammon telling the judges before they auditioned that there was "exactly one year" between them, prompting Katy to reply: "I used to wish for twins, and then I finally had a child, and I go, (that's) crazy."

The pair then added that they were the oldest of eight children, leading to Katy looking mortified.

Liahona and Ammon performed an original song for the panel, which scored them two tickets to the Hollywood segment of the competition.

Katy's admission about wanting twins comes just weeks after she credited baby Daisy for changing her life.

"I think that you realise that when you become a mother... you just have to focus on being a mom. And it's not because you don't love other people, it's not because of anything besides you just want to be a great mom," she said on Instagram Live in January.

"So a lot of stuff falls away when you become a mom and it is the best job in the world. I highly recommend it when you're ready."