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

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.''