A new government report finds women with children under 18 years of age earn less than women without young children, while men with kids under 18 not only earn more than the women, but they also bring in more than men without younger children.

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Some research even suggests that women generally make less money for each child they have, while men are more likely to make more money for being married and having kids.

So, why is that?

"There appears to be a motherhood penalty for women in the workplace," said Terri Boyer, executive director of the Center for Women and Work at Rutgers University. "There are a lot of explanations for the wage gap and they are many and complex. But no matter how many things we try to chalk up to what is going on here -- like women choosing fields that pay less or women being more likely to work part time -- you still find a wage differential, about five to seven percent, that can't be explained through anything besides discrimination or a lower value of women in the workplace."

The report by the Bureau of Labor Statistics found that in 2012, women with children under 18 earned $680 per week, while women without children under 18 brought in $697 per week. On the other hand, men with kids under 18 had median weekly earnings of $946 compared with $799 for men without minor children.

"I think one of the forces of change that will be important in this equation is fathers," said Boyer. "As we see more fathers coming into the workplace and taking a stronger role in parenting their children and asking for flexibility to accommodate, you're going to see an equalization force here.

"It's also important for employers to understand that mothers who are in the workplace are choosing to be there and need the accommodations necessary, and should be as valued and paid in equal amounts as their male counterparts or their female counterparts who don't have children."