Gisele Bündchen is "so grateful" for her step-son Jack, whom she calls her "bonus child".

Gisele Bündchen

Gisele Bündchen

The 38-year-old supermodel had her relationship with now-husband Tom Brady rocked in 2007, when just two months into their romance they discovered that Tom's ex-girlfriend Bridget Moynahan was pregnant with the sports star's first child, a son named John 'Jack' Edward Thomas, now 11.

And although the situation wasn't "ideal" for anybody involved, Gisele - who now also has son Benjamin, eight, and daughter Vivian, five, with Tom - "couldn't imagine" her life without the youngster.

She said: "It wasn't the ideal situation for anyone - there's no manual to prepare you for that. But I'm so grateful for [Bridget]. I know this was hard, but I couldn't imagine my life without [Jack]. I call him my bonus child. He opened my heart in ways I didn't even know my heart could expand."

Watching Tom and Jack bond made Gisele want her own children with the NFL star - whom she married in 2009 - and although she knew she wanted to become a parent, she says a "part of her died" when she had her first child.

She added: "When I became a mom, I kind of lost myself. It was like a part of me died. I'd been this very independent person. It was all about me. But now I had this little being, and I suddenly felt l couldn't do other things and that was very hard for me. All I ever wanted was to be a mom, but when you're actually experiencing that, it's a shock."

Gisele is gearing up to release her new memoir, 'Lessons: My Path to a Meaningful Life', and has said she decided to pen the tome because she thought it was "time to share" her "vulnerabilities".

Speaking to People magazine, she said: "Things can be looking perfect on the outside, but you have no idea what's really going on. I felt like maybe it was time to share some of my vulnerabilities, and it made me realise, everything I've lived through, I would never change, because I think I am who I am because of those experiences."

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