Leviticus 12:4,5

The time of purification after the birth of a female was twice as long as that after a male. The belief that the birth of a girl was more dangerous for the mother than the birth of a boy prevailed among ancient nations, who considered that hostile supernatural beings were more to be feared in these cases. The practice, at here, survived the belief on which it was founded. Observe that the mother is regarded as unclean but not the child.