Source : Webster's Revised Unabridged Dictionary (1913)

Afterbirth \Aft"er*birth`\, n. (Med.)
   The placenta and membranes with which the fetus is connected,
   and which come away after delivery.

Source : WordNet®

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