Leviticus 12:5 — why is the seclusion twice as long for the birth of a female child as for the birth of a male child?
A number of explanations are given for the doubled period of ‘seclusion’ (impurity of Niddah) for a female birth, as well as for the double period of purity (66 days rather than 33) that follows the female birth.
One explanation is that the basic formation of a female takes twice the time of the formation of the male (see Ibn Ezra, Lev. 12:4; Ramban 12:4). This is based on one opinion of the Mishnah (Niddah 3:7).
Another expanation is that the physical changes of the mother are greater for the birth of a female than for the birth of a male, and the time of her impurity is therefore greatened (see Ramban, ibid.)
A third explanation relates the experience of birth to the initial creation of the man and woman. Whereas Adam was created only once, Eve underwent two processes of creation: one as a part of Adam, and another as a distinct, independent entity. Therefore, the periods of impurity and purity are doubled for a female, expressing her ‘doubled’ creation.