Gwen Stefani running out of time for kids

Gwen Stefani thinks she is "running out of time" to have another child.

The 40-year-old singer – who has sons Kingston, three, and 18-month-old Zuma with husband Gavin Rossdale – has mixed feelings about expanding her family but knows she needs to make a decision soon.

She said: "No one could have prepared me for how awesome motherhood is, and how hard. I often feel like I'm running out of time. But I don't want to spread myself so thin that I can't be good for them. Poor Zuma, he'd be the middle guy.

"I'm not focusing on it right now, but it's not up to me anyway, you know? These things are miracles, so we'll see."

Despite their young age, the No Doubt star is amazed by how different her two children are already.

She added to America's InStyle magazine: "It's crazy how the second child is so different from the first, and has the same parents. They are totally different, and they look totally different.

"Zuma is more of a show-off. He wants everyone to watch him, and he's a goofball and he's really silly. He's a funny guy."