Unfortunately I think in many companies being a mother is still seen as a limitation regardless of how it impacts your job performance. Employers naturally assume that as a mother you don't give your full attention to your work activities. Hopefully with time the perception will change. At my last job, male colleagues in the same position were making about 20% more than me. I realized that I didn't want to work for a company that didn't value me, so I left and explained why in my exit interview with HR.
I also think part of the reason that women aren't making as much as their male counterparts is because we aren't good at negotiating for money. I always feel like I am at the losing end of the stick, partially because many times I don't know what to ask for. A male colleague of mine got a golf membership thrown into his benefits; I would have never even known to ask.
I'm currently reading a book I'd recommend on the negotiating subject - The Good Girl's Guide to Negotiating