Katy Perry filmed her sister's birth on her mobile phone.

The 'Roar' hitmaker was by her older sibling Angela Hudson's side last week when she welcomed her little girl into the world, and although she helped deliver her niece 29-year-old pop star still managed to capture the moment on film.

Katy - real name Katherine Hudson - said: ''My sister had a baby in her living room - very naturally, not one Advil. I was there kind of being an assistant doula and saw everything.

''We had dinner first, obviously before I saw it all. No, it was just there and it was an incredible experience.

''I was kind of like Martin Scorsese with my iPhone - I have the whole thing. It was like a water birth, the whole thing we're like 'The child is swimming through the water.' It's crazy.''

After witnessing a live birth, Katy went on to hail mothers as ''real rock stars''.

The 'Dark Horse' singer - who recently split from boyfriend John Mayer after 18 months of dating and was previously married to Russell Brand - said: ''It's insane, like birth and women giving birth - whenever I see moms walking down the street I'm like, 'Damn, you're a rock star, you're the real rock star - you did something incredible.' ''

And Katy admitted her entire family are excited by the arrival of Angela's baby, especially her parents Keith and Mary Hudson.

Speaking on the Fifi & Dave Show on Australian radio station Fox FM, she said: ''The baby girl is so alert and aware, and active and incredible. It's the first time in our family to have a little one so everyone's very excited.''