"Kekkais are to be killed!" was the Shogun's order. He called the deformed infants "Kekkai". And it hasn't changed for many many years. Officials were tasked to take those newborns away to execute them. Mothers would weep, put up fight in order not to let go of their infants. Fathers would be helpless, abiding the rules in agony. They say, the Shogun didn't even spare the Emperor's nephew who was born a Kekkai.
Anyway my children.. that night another "Kekkai" was born.