New Defender's Study Bible Notes
12:5 maid child. The purification period after the birth of a son was forty days, but eighty days in the case of a daughter. The reason for the difference is not given, but possibly had to do with two considerations. It was the woman who had originally been deceived, with the resultant pronouncement of travail in childbirth (Genesis 3:16), with the additional purification period symbolic of the future travail the daughter must also anticipate. The second factor could be that the rite of circumcision, applicable only to the male child, symbolized the more direct entrance into covenant relation with God. See also note on Genesis 17:11.
12:5 unclean two weeks. Another possible reason for these seemingly rigorous laws of purifying for a new mother is that the pagan nations around them are said to have had similar though much more severe regulations required of their own women. The Hebrew regulations were much gentler in comparison but were retained in this milder form in order to keep from offending these Gentile neighbors unnecessarily. At least this explanation was offered by medieval Jewish theologians.