Why is the length of the Nidah different after giving birth to a boy or a girl?
Let us first clarify the facts, and then offer possible explanations. After the birth of a boy, the woman is impure for 7 seven days (even if there was no blood), and pure for the following 33 (even if she were to see blood). After the birth of a girl, the woman is impure for 14 days, and pure for the following 66. Thus, both the impure and pure days for a girl are doubled.
Nowadays, we do not practice the "pure" days. The woman remains impure and has the status of a "nidah" until the bleeding stops and she can count the seven clean days, for both a boy and a girl. Afterwards, she may immerse in the mikvah.
Therefor, there is no practical difference nowadays between the pirth of a boy and that of a girl.
Why are the days doubled? Some early commentators (see Ibn Ezra and Ramban Vayikra 12:4-5) suggest that there is a biological difference between the creation of a female and that of a male, or in the quantity of secretions that follow the birth. This explanation is difficult to accept nowadays, although there may be deeper, kabbalistic, differences in the formation of their respective spiritual nature. Another explanation, offered by Rav Shmuel Eliyahu shlita, is that Adam underwent a single creation, whereas Eve underwent a double creation – first as part of Adam, and afterwards as a distinct person. Thus, the birth of a girl represents a double birth, and hence the days of impurity and purity are doubled.A third explanation can be offered based on Chazal's comment that bris milah is on the eighth so that the couple would be permitted to each other and share in the joyful occasion. Thus, it is possible that the Torah halved the impure and pure days for the birth of a boy to conform to the day of the bris.In a similar vein, one can offer an additional explanation. R. Meir (Nidah 31b) teaches that the separation required during the days of nidah is so that the husband should not view their relationship as routine, but should cherish his wife with renewed joy after her immersion. Thus, after the birth of a girl, we require a double waiting period so that the husband's subsequent joy will parallel the joy accompanying the birth of a boy.Rav Meir Orlian