In Leviticus 12 it says that the woman who gave birth to a boy is only unclean for seven days, but one who gave birth to a girl is unclean for two weeks. Why is that?
"Speak to the children of Israel, saying: 'If a woman has conceived, and borne a male child, then she shall be unclean seven days; as in the days of her customary impurity she shall be unclean. And on the eighth day the flesh of his foreskin shall be circumcised. She shall then continue in the blood of her purification thirty-three days. She shall not touch any hallowed thing, nor come into the sanctuary until the days of her purification are fulfilled. But if she bears a female child, then she shall be unclean two weeks, as in her customary impurity, and she shall continue in the blood of her purification sixty-six days. When the days of her purification are fulfilled, whether for a son or a daughter, she shall bring to the priest a lamb of the first year as a burnt offering, and a young pigeon or a turtledove as a sin offering, to the door of the tabernacle of meeting'" (Leviticus 12:2-6).
It was because of the blood flow that occurs during the birth of a child that a woman became unclean. However, since a male child was circumcised on the eighth day, that sacrifice of the foreskin with its blood flow was considered "payment" for the second week of uncleanness, and mom only needed 33 more days for purification. Because females did not have a circumcision rite, the woman remained unclean for two weeks and needed 66 more days for purification.
Or another way to look at it, in a society where men valued boys, mom got to rest an extra forty days when she had a girl.