The couple, who married in 1999, are the parents of three sons -- Brooklyn, Cruz and Romeo.
Their baby girl is due to be born in July.
"Obviously, we're very lucky to be expecting again, and this is the first time I'm going to say it: It's a little girl," the Daily Mail Friday quoted the LA Galaxy midfielder as saying. "We're still in shock. Obviously, having three boys, you kind of expect another one, so finding out a little girl is in there is surprising, but, obviously, we are over the moon. ... Our three boys are happy and excited, and Victoria is doing well."
Aaron Carter is still in love with Hilary Duff
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