The two greatest obcenities in the society of Brave New World are birth and mother. Why?

I really think you have it there. Certainly the greatest natural bond in life is between mother and child. This is very hard to break. Even in Orwell's 1984, the party had to begin their indoctrination process from infancy. Emotional attachments lead to internal meaning. This meaning, between mother and child, defines much of our personality and how we see the world. The party in Brave New World functions on superficial attachments. The mother/child bond must hence become an "obscenity".