Answer:
   
   A woman who gives birth has the status of a Niddah (a menstruant) who
   is both ritually impure and off-limits as far as intimate activity to
   her husband. The act of giving birth induces a special nidus state
   unrelated to the uterine bleeding that normally separates husband and
   wife. This special nidus extends for seven days after the birth of a
   son and fourteen days after the birth of a daughter. Since it is most
   unusual for a woman to stain for less than two weeks after childbirth,
   this special nidus has little practical application, except in some
   cases of cesarean delivery. The ritual after the end of the period is
   the same as in normal Niddah status: immersion in a mikveh.
   
   During the time of the Beis HaMikdash (Holy Temple) there was an
   obligation to bring a korban (offering) forty days after the birth of
   a boy or eighty days after the birth of a girl. As to why the
   purification for the birth of a female is longer, the belief is that
   this is because since a female herself is capable of producing life,
   she is capable of imparting more ritual purity as well as impurity.