As Parshat Toldos begins Rivka is having a difficult pregnancy, her children are fighting inside her. Rivka asks, Im Ken Lama Zeh Anochi, a phrase which can be translated in a number of meanings. But the most literal is, If So Why Am I? Commentaries tell us that Rivka had despaired of her life entirely. G-d then informs her that there are two quarelling nations inside her and that one will go on to oppress the other. How is this remotely reassuring to tell a worried pregnant woman that this state of affairs will continue on into their adulthood and to their descendants as well? It would seem that there could be little better formula for despair for a mother to know her children will hate and fight each other forever.
But let us first consider why was Rivka so upset? Was it merely because she had a difficult pregnancy? It doesn't seem that a difficult pregnancy would have driven her to such measures. Her question seems to suggest that the two children fighting inside her makes her existance.
Rivka had married Yitzchak and in doing so inherited the burden of expectations of the family of Avraham whom his father had blessed with the blessings G-d had given to him. The blessings had gone to Yitzchak rather than to his older brother by another mother who was wicked and undeserving of them. Rivka could recognize on her own and did, that her two children were fighting. What this meant to her however was that instead of producing another Yitzchak as Sarah had, whose place she had taken, she was producing one or more wicked children who were unfit to carry on the heritage of Avraham. Rather than being another Sarah, she was another Hagar.
G-d's message however told her differently. G-d emphasized that there were two nations both inside her womb and that would be born outside it and that one would prove superior. This meant that first her children were not fighting because they were both wicked and liked to fight, but because they represented two ways of life, only one of which could prevail. Her mission was not simply to bear children for Yitzchak but by her actions to choose which would prevail.
When Avraham had to choose the son who would carry on his legacy, it was Sarah who pushed him towards expelling Yishmael whom he had fondness for in favor of Yitzchak. This too would be Rivka's mission, to sway Yitzchak towards Yaakov over Esav whom he favored. Similarly Leah and Tamar would be forced to use extraordinary and sometimes deceptive measures to sway Yaakov and Yehuda to see that the future of the Jewish people would be born. Even down to King David's father and mother a similar event had to take place.
To this day while the father determines the blessings of tribal ancestry, e.g. to be a Kohen or a King, it is the mother that determines whether the child is fit or not fit to recieve for them the child of a non-Jewish mother remains non-Jewish while the child of a Jewish mother is Jewish regardless of the father. It is through her that the child's future comes about.