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.