The 37-year-old's toddler was diagnosed with Transposition of the Great Arteries (TGA), a rare heart defect in which the two main arteries of the heart are reversed, when she was 20-weeks pregnant.

The tot underwent life-saving open-heart surgery at Los Angeles' Cedars-Sinai Medical Centre recently, with Jaime taking to Instagram late last month (Aug16) to thank the medical staff for their work in saving her 14-month-old son's life.

Now, the Sin City star has shared an update on Leo Thames' health, telling he is "doing amazing" as she opened up about her "terrifying experience" seeing her son undergo the critical operation.

She recalled Leo needed to be a certain weight just so he could survive the surgery, and confessed her son's condition was so serious, they nearly lost him.

"I was wheel-chaired to him every three hours, so I could breastfeed him and take care of him before he went into this huge surgery," she said. "It was a terrifying experience. But thank God for the medicine that we have now."

Jaime went public with her story after Taylor Swift, her best friend and Leo's godmother, made a donation to the medical centre after her boy's surgery. The singer was one of the only people outside the actress' immediate family who knew about Leo's health struggle.

The newly-single Shake It Off hitmaker made the donation on behalf of Leo, and Jaime said she decided to make the gift public to honour the doctors and nurses who treated her son.

The actress was compelled to share the reality of her son's health struggles because she "knew how traumatic the experience was, how much (post-traumatic stress disorder) I had afterwards, and the trauma that I was experiencing before it."

"It's because I didn't know anybody that had gone through it and I didn't have people to talk to," she added. "I have a voice and I want to use that voice on behalf of my son - and I know that my son would want that. I've gotten thousands of letters from people and now I have a real community of people to talk to."

King wed filmmaker Kyle Newman in 2007, and they are also parents to two-year-old son James Knight.