Anna Faris Opens Up About Son Jack's Health Battle in Revealing New Book: 'It Was Us Against the World'

Anna Faris and Chris Pratt's 5-year-old son Jack is a healthy little boy today. But there was a time when the proud parents were unsure of their son's fate.

In her revealing new memoir Unqualified, the 40-year-old actress details Jack's birth and the "emotionally exhausting" weeks that followed, in which she and now-ex Pratt worked to come to terms with terrifying news.

"The pediatric neurosurgeon sat Chris and me down to tell us that Jack had some severe brain bleeding and there was a chance that he could be developmentally disabled," Faris writes of Jack, who was born several weeks premature.

"I was in complete shock … So Chris and I did what we could, which was hold hands and hope and face it together," adds the Mom star.

Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.

RELATED VIDEO: In Their Own Words: Chris Pratt & Anna Faris On The Highs & Lows Of Their Relationship

Doctors told the parents that they would not know for sure until Jack was about 18 months old whether he would have special needs. Faris had been on bed rest for about a week before going into labor.

As doctors swarmed and she confronted the pain of not knowing when she'd get to hold her baby, Faris says her mindset became, "I have to be as strong as I possibly can … I have to."

But the hardest moments for the couple came after the birth, as Jack was poked and prodded in the neonatal intensive care unit for weeks.

"These moments can be hard on couples, but for us it really brought us together," she writes. "It felt like it was us against the world."

About a month later, Faris and Pratt, 38, got to take Jack home. And in the years that followed, the little boy would grow healthy with only slight vision and leg muscle issues.

"Today, Jack is at a beautiful age. He has some temper tantrums occasionally (did I mention he's 5?) but he's a really good kid and he's happy and delicious and likes cuddles," the proud mom continues.

FROM PEN: Grey's Anatomy's Chandra Wilson Opens Up About Daughter's Mysterious Vomiting Syndrome — and There's No Cure

Jack has been Pratt and Faris' main focus since the former couple split; they announced the breakup in August. Now, she tells PEOPLE that she's grateful for Pratt and their greatest treasure: Jack.

"We have the most amazing little boy together," Faris says.

Unqualified is available starting Tuesday.

For more about Anna Faris' soon-to-be-released memoir, pick up the latest issue of PEOPLE, on newsstands Friday.