The mum-of-three, who gave birth to her third son Kit in February 2016, uploaded a collage of eight photos of her baby son going from frowning to giggling as he gazed ahead.
"Looking at himself in the mirror 😂😍," Rooney captioned the snap.
Rooney, who is also mum to Kai, six, and Klay, two, hasn't shared many photos of her newborn since his arrival.
The latest snaps show baby Kit has a head of fair hair unlike his brothers.
Rooney uploaded a photo of Klay kissing his younger brother at the beginning of March 2016.
"I love my brother 👦🏼👶🏼," she captioned it.