Левит 12:5

But if she had given birth to a girl, she was to be unclean two weeks (14 days), as in her menstruation, and then after that to remain at home 66 days. The distinction between the seven (or fourteen) days of the "separation for her infirmity," and the thirty-three (or sixty-six) days of the "blood of her purifying," had a natural ground in the bodily secretions connected with child-birth, which are stronger and have more blood in them in the first week (lochia rubra) than the more watery discharge of the lochia alba, which may last as much as five weeks, so that the normal state may not be restored till about six weeks after the birth of the child. The prolongation of the period, in connection with the birth of a girl, was also founded upon the notion, which was very common in antiquity, that the bleeding and watery discharge continued longer after the birth of a girl than after that of a boy (Hippocr. Opp. ed. Khn. i. p. 393; Aristot. h. an. 6, 22; 7, 3, cf. Burdach, Physiologie iii. p. 34). But the extension of the period to 40 and 80 days can only be accounted for from the significance of the numbers, which we meet with repeatedly, more especially the number forty (see at Exo 24:18).

After the laws concerning clean and unclean food come the laws concerning clean and unclean persons; and the first is in this chapter concerning the ceremonial uncleanness of women in child-birth (Lev 12:1-5). And concerning their purification from that uncleanness (Lev 12:6, etc.).

12:5 two weeks . . . sixty-six days: The total is eighty days, double the time required for the birth of a son (see also 27:2-7).