Kate Ferdinand has battled with her mental health and insecurity over her changing body during her pregnancy

The 29-year-old author usually conquers her anxiety by keeping active, and when she hasn't been able to workout, it's impacted on her mental health.

Kate - who is a stepmother to Rio's kids Lorenz, 13, Tate, 11, and nine-year-old Tia - also admitted to feeling "guilty" when she's relaxing.

In an interview with Women's Health's 'Going for Goal' podcast, she admitted: "I struggle with my mental health ... being active helps me day-to-day with my anxiety ... So when I can't do things, I really struggle with it ... I want to do everything. Rio says to me 'Kate, just sit down. You just need to sit down and relax'. But I don't know why, I just feel guilty when I do.

"[Anxiety is] something I've suffered with from when I can remember. It can start over the simplest thing, and it's just my mind playing tricks on me and can get out of control. But I do find that when I exercise or go for a walk, I seem to feel better for the day."

On how her mental health has been during her pregnancy, she added: "I am quite emotional. I feel a little bit insecure ... I feel like not like me, and I struggled with that for a little while...feeling insecure with just how I look and my changing body, and I know it sounds very superficial ... but struggling how to dress for this ... Everything that you know has changed."

However, Kate admitted she felt "a bit more normal" by the second trimester.

She added: "The first three months I really, really struggled. I'm normally really active and I run around like a lunatic, just doing everything constantly. And I just couldn't ... I couldn't get my head around being tired and not having the energy to go to the gym ... But as I'm in the second trimester, I feel a bit more normal ... I feel a bit more like myself."

Listen to the full Kate Ferdinand interview on the Women's Health 'Going for Goal' podcast, available now.