The mum-of-one welcomed her only child, Jack, five years ago with then husband Chris Pratt.
But now the mum has described how difficult the first few weeks of motherhood were, after their little boy was born several weeks premature.
In her new memoir, Unqualified, the actor has revealed that her son Jack was born with severe brain bleeding and there was a chance he could be 'developmentally disabled'.
Describing the moment doctors told her and Chris about their son's health problem in an exclusive extract of the book obtained by People magazine, the mum said she was blindsided by the news.
'The paediatric 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', Anna said of the moment they were first told their son was ill.
Anna and Chris welcomed their son Jack in 2012
The Scary Movie star said she was in 'complete shock', but the difficult period when Jack was being treated in the neonatal intensive care unit also brought her and Chris closer together.
'I was in complete shock. So Chris and I did what we could, which was hold hands and hope and face it together', she writes in her new book.
'I have to be as strong as I possibly can. For us it really brought us together. It felt like us against the world.'
Thankfully Jack was brought home from the hospital by his loving parents a month later, and has since had a mostly healthy childhood, having only developed minor issues with his vision and leg muscles.
Despite having gone through the tough ordeal together, Anna and Chris have recently announced they are going their separate ways after ten years together.
Jack is now five years old
In August this year, eight years after they tied the knot, the pair took to social media to tell fans that they were legally separating, but that they still 'love each other'.
'We are sad to announce we are legally separating,' the couple, who are parents to four-year-old Jack, wrote. 'We tried hard for a long time, and we're really disappointed', they wrote in their statement.
'Our son has two parents who love him very much and for his sake we want to keep this situation as private as possible moving forward.'