And she shall then continue in the blood of her purifying three and thirty days; she shall touch no hallowed thing, nor come into the sanctuary, until the days of her purifying be fulfilled. (Leviticus 12:4)
The first six days following childbirth were critical for the mother, and often there was a considerable loss of blood. After a week the crisis was supposed to be past. For 33 days the mother was not permitted to come into the sanctuary or to participate in religious ceremonies. She was not to attend any public gathering. It was the mother, and not the child, who was considered unclean.