So by giving birth they lose their magic, and the daughter has no guarantee of being a witch? Damn.
Also, if you assume they can give birth to boys too, no wonder they were pushed into a corner.
The question would be, can one witch give birth multiple times and each daughter has a chance? Or the magic transferral is lost forever after the first child?
I mean, not like it's an enviable existance, but if they can... Once a witch decides, or is selected, to have a child, she "just" to keep pumping out babies.