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I understand that any ideas or research presented on this matter will keep the company of "hypothetical" since, even the Jewish sages are divided and uncertain.
What we know is that if a woman gave birth to a male child, she was unclean for seven days and then, upon the eighth day, the male child was to be circumcised. Then, 33 days following this the woman would still not be allowed into the Sanctuary thus placing her process of purification at 40 days.
However, if a female child was born, the mother is regarded unclean for 14 days (twice the amount) and then her period of time not being allowed to enter the Sanctuary was 66 days (once again, double that of the male child). This put her total time of purification at 80 days.
Leviticus 12:1-8 (KJV) 1 And the LORD spake unto Moses, saying, 2 Speak unto the children of Israel, saying, If a woman have conceived seed, and born a man child: then she shall be unclean seven days; according to the days of the separation for her infirmity shall she be unclean. 3 And in the eighth day the flesh of his foreskin shall be circumcised. 4 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. 5 But if she bear a maid child, then she shall be unclean two weeks, as in her separation: and she shall continue in the blood of her purifying threescore and six days. 6 And when the days of her purifying are fulfilled, for a son, or for a daughter, she shall bring a lamb of the first year for a burnt offering, and a young pigeon, or a turtledove, for a sin offering, unto the door of the tabernacle of the congregation, unto the priest: 7 Who shall offer it before the LORD, and make an atonement for her; and she shall be cleansed from the issue of her blood. This is the law for her that hath born a male or a female. 8 And if she be not able to bring a lamb, then she shall bring two turtles, or two young pigeons; the one for the burnt offering, and the other for a sin offering: and the priest shall make an atonement for her, and she shall be clean.
Why the "doubling" between male and female? Both activities of labor brought about a ritual uncleanliness due to menstrual blood, but why the doubling? Is there something scientific between male and female children that would account for this? I know that minor "menstruation" occurs with female infants due to the hormones she is exposed to while in her mothers womb. Interestingly, many say that infant menstruation lasts roughly 1-2 weeks maximum. Could this be the reason? Is the purification double because it is occurring for both the mother and the daughter?