Srimad-Bhagavatam 1.11.29 Purport
According to scriptures also, there are seven mothers: (1) the real mother, (2) the wife of the spiritual master, (3) the wife of a brähmana, (4) the wife of the king, (5) the cow, (6) the nurse, and (7) the earth. All of them are mothers. Even by this injunction of the sästras, the stepmother, who is the wife of the father, is also as good as the mother because the father is also one of the spiritual masters. Lord Krsna, the Lord of the universe, plays the part of an ideal son just to teach others how to treat their stepmothers.