Is it possible that the reason a woman is tamei for 66 days after giving birth to a girl is because not only the mother but also this baby can be a nidda as mentioned in note 10 on 55B2 in Artscroll?
MICHAEL PLASKOW, Netanya - Israel
I believe you meant to ask, is it possible that this is why she is impure for fourteen days instead of seven (as the sixty six days are days when her blood is considered *pure*, not impure).
While I have not seen this exact answer, there are explanations on the Pasuk in Tazria (12:5) that explain why impurity gravitates towards women more frequently than men. [For example, see the Kitzur Alshich on the Pasuk above.] This includes, in part, a baby girl drawing impurity more than a baby boy, and the fact that a baby girl can indeed become a Nidah.
All the best,