I have answered this question before (and you can google for a much better answer than I have time to give right now. The risk comes (if risk there is) if blood from the mother accidentally gets to the blood of the baby (or, actually vice versa).
This can happen during birth for a number of reasons (and I'm not going to get too graphic here).
The problem is also easily fixable if it is recognised as existing.
Google is your friend, and the explanations are easy to understand.