German supermodel Heidi Klum is finding it hard to lose her baby weight.

Heidi, who gave birth to daughter Lou last month admits she has struggled more to get back into shape than she did after having her other children, Leni, five, Henri, four, and Johan, three, reports contactmusic.com.

"I wouldn't say I'm back in shape yet. Everything changes when you get older. It was different when I was 31 with the first one and now being 36 with the fourth one," she said.

Heidi is now hitting the treadmill and eating sensibly in a bid to get back in shape as she fears she will remain out of shape forever if she doesn't act now.

"I think that if you wait too long and it sticks there it stays that way forever. I have to be motivated," she said.