Someone asked me why women were considered unclean after giving birth? Why were they considered unclean longer after giving birth to a girl? Is that a dim reflection on females?
I'm not sure if the uncleanness of a mother for a female baby was extended. Maybe the uncleanness of a mother for having a male child was "cut short" for the sake of circumcision - no pun intended! Mom would need the all clear by the eighth day in order to participate in presenting the child to the priest. The fact that the offering requirements presented to the temple were the same for male or female infants indicates that there was no greater value placed on one gender over the other.
Another possible reason for giving the newborn females more time away from the populous could be health related.
"...mortality differentials arise from social or behavioral factors reflecting deliberate discrimination by adults in favor of boys over girls, resulting in atypical male to female infant mortality ratios."
- Kana Fuse and Edward M. Crenshaw; Department of Sociology, The Ohio State University, July 2005
This quote, which applies to underdeveloped cultures, is saying that such cultures have a tendency to place fess value on female babies, contributing to higher female infant mortality. The extra week would give a longer buffer for more intense care of such infants. Those extra few days of isolation, just mother and baby, could have also played a factor in the overall health of the baby, by allowing preemptive quarantine (childbirth fever, also known as Puerperal fever was highly contagious and prevalent as recently as the 1800's) as well as more bonding time for mother and child.