Ellenpoly - The best explanation, so far is that the the holy spirit is the life-giving force. That makes sense because in those days, conception was a total mystery. It is also the mediating principle between God (the heavenly father) and the Son (Jesus). It would stand to reason that the holy spirit is the mother. In that case mother and father and child represent the triniity (joined together as one in God).

Thats the best I've come up with so far.