In an interview with Press Association, she explained that her previously strong faith was shaken while she was longing for a family: "My faith's always sustained me through tough times like divorce and some of my bouts of black depression.
"But my worst time was when I was desperate for children, having IVF but it failed three times.
"Disappointment doesn't cover what that felt like - it was like a black hole opening up."
Fern Britton described herself as "an average Church of England Christian" and said that after the final failed attempt she almost stopped believing.
"At that point, I just said to God, 'I don't believe in you anymore, so I won't have to worry about my conscience or judging right from wrong'. It turned out I could only keep that up for half a day.
"My faith's too ingrained in me and, shortly after that, the fourth course of IVF finally worked and I had the twins when I was 37.
"I'm so grateful for everything I have now, Phil and the kids and such a happy home life."
Fern Britton already had three children from her first marriage.
She is now married to TV chef Phil Vickery and will return to TV presenting For What It's Worth, a new antiques strategic quiz show.