The Collaborative International Dictionary of English v.0.48:

Afterpains \Aft"er*pains`\ ([a^]f"t[~e]r*p[=a]nz`), n. pl.
   (Med.)
   The pains which succeed childbirth, as in expelling the
   afterbirth. AS
   [1913 Webster]

WordNet (r) 3.0 (2006):

afterpains
    n 1: pains felt by a woman after her baby is born; associated
         with contractions of the uterus