Don't know much of historic significance, but it seems to me this could be a form of revenge. The children of Jewish mothers are Jewish themselves (by Jewish law and/or tradition). Therefore, raising the bastard children of their tormentors as Jews might have a certain satisfaction. It also might make them reluctant to abort a child that will be a Jew.
That's a very interesting idea. How often do you think women in general raise children out of vengeance?