Jessica Simpson ''doesn't understand'' how she got pregnant again.

The 38-year-old singer - who already has Maxwell, six, and Ace, five, with husband Eric Johnson - is expecting her third baby but admitted she ''didn't really know'' whether or not she could conceive again and was stunned when she did because she and her spouse hadn't been particularly busy in the bedroom.

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.''

The 'Dukes of Hazzard' star's older children are very excited about having a younger sibling, but Eric, 39, has warned his wife to stop being ''so honest'' when it comes to answering their questions.

She told 'Entertainment Tonight': ''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.''

The 'Blonde Ambition' actress took to Instagram last month to share the happy news, when she posted a picture of her two children surrounded by pink balloons - which revealed the gender of Jessica's unborn baby.

She wrote in the caption: ''This little baby girl will make us a family of five. We couldn't be happier to announce this precious blessing of life.''