Jessica Simpson is ''working really hard'' on her post-baby body.

The 39-year-old singer welcomed daughter Birdie into the world three months ago and she had been putting a lot of focus on her figure.

She told People magazine: ''I am working really hard right now. It's not easy at all, but I am determined to feel good. I have been doing a lot of walking - getting my steps in not only burns calories but it also helps me clear my head and get focused. I am eating healthy too - I discovered I really like cauliflower. Who knew it could be a substitute for almost anything?''

Meanwhile, Jessica - who also has Ace, six, and Maxwell, seven, with her husband Eric Johnson - previously confessed she ''doesn't understand'' how she got pregnant again.

She admitted: ''I thought about [having another baby], but I didn't really know that it could actually happen. We definitely always love to practice. We actually weren't practicing very much that month so I don't understand. We try [to figure it out]. This might be a miracle baby.''

But despite her confusion over the new arrival, her children were ''so excited'' to be a big brother and big sister.

She added: ''They're so excited because they're only fourteen months apart, so they didn't get to experience the excitement of each other. But when I told them that we're having another baby, I know that they'll remember that for the rest of their lives. They are most excited about learning how to do the swaddle. Maxwell, my daughter is just so happy that it's a girl. They ask me the craziest questions that I can't say on TV ... 'How does the baby come out?' 'Where does the milk come from, I don't understand!' I'm a little bit too honest, and Eric's like, 'I don't think we need to give out ALL the information, Jessica. So, I'm scaring my kids, I think.''