The fitness queen documented the process in a series of Instagram Stories on Sunday.
The 31-year-old and her fiancé headed to the hospital, where Kayla underwent a planned C-section birth.
“Words absolutely cannot describe this feeling. We are SO HAPPY RIGHT NOW,” she wrote in her caption.
“We went into the hospital around 6.30 am on the 5th of January. He was out in the world by 8.45am weighing 3.5kg.”
Itsines went on to share intimate details of the birth, which was a planned c-section.
“The most amazing thing, as he was coming out, out of nowhere on a completely random playlist the hospital was playing, (partner) Jae’s dad's funeral song started playing over the speakers,” she said.
“I heard Jae say at the same time ‘It’s a boy!!!!!’ I burst into tears, he burst into tears and we got to meet our baby BOY.”
She then revealed her son’s name, writing: “Jax Stokes Woodroffe. Healthy and quiet”.
Earlier that day, Itsines began documenting the process of the birth in a series of Instagram Stories, as she and her fiancé headed to the hospital.