The Sages said, “Children, life, and sustenance don't depend upon one's merits, but upon one's luck – Mazal.” Mazal, however, is not chance or luck, but rather a spiritual force. In fact, there are two kinds of Mazal: Mazal-8, called Isaac, and Mazal-13, called Rebecca. When the two unite, they represent complete Mazal. There is a strict aspect of Mazal, and the union of Isaac and Rebecca was fruitless. It is this strict Mazal that Isaac overturned with his prayer, and Rebecca became pregnant.
Rebecca came as a reincarnation of Eve. Her children fought in her womb, and Esau, who was a reincarnation of Cain, wanted to break his brother's skull. Rebecca therefore asked “Why am I?”
Rebecca's sons were fighting, just like Eve's sons were fighting. Esau wanted to kill his brother, only this time he started doing this earlier than in his previous life as Cain. This did not seem an improvement of Eve's problem.
She was told by the prophet that her two children will “separate” - that is, they will always fight, one of them drawing all the holiness to himself, and that in the end “...the big one will serve the small one...” This represented precisely the rectification of what Adam had done by mixing the good and evil together.