Researchers used LDS genealogical data to measure how many years kids take off their parents' lives. They looked at 21,684 couples married between 1860 and 1895 to determine the "fitness cost" of human reproduction.
Older mothers were four times as likely to die in the year after having a child than their mates. Having lots of children was especially risky. A mother of 12 had five times the risk of dying prematurely as a mother of three.
The price of parenthood wasn't trivial for men, either. Despite the obvious fact that men avoided the hazards of childbirth, fathering more children meant more risk of dying before their time, too. [Link]