The Collaborative International Dictionary of English v.0.48:

afterbirth \aft"er*birth`\ ([a^]f"t[~e]r*b[~e]rth`), n. (Med.)
   The placenta and membranes with which the fetus is connected,
   when they are expelled from the womb after delivery.
   [1913 Webster + AS]

WordNet (r) 3.0 (2006):

afterbirth
    n 1: the placenta and fetal membranes that are expelled from the
         uterus after the baby is born