Jacob was born 20 years after Isaac and Rebekah were married, at which time his father was 60 (Gen. 25:26), and Abraham, 160 years old. He and his twin brother, Esau, were markedly different in appearance and behavior. Esau was a ruddy hunter, while Jacob was a gentle man who "dwelled in tents," interpreted by most biblical scholars as a mark of his studiousness in the "tents" of Torah.
During Rebekah's pregnancy, "the children struggled together within her" (Genesis 25:22).
According to Rashi, whenever Rebekah passed a house of learning, Jacob would struggle to get out; whenever she passed a house of idolatry, Esau would struggle to get out.
Fearing that she was carrying one rather schizophrenic child, Rebekah questioned God about the tumult and learned that two children were in her womb, who would become two very different nations.