Why did so many young women die in the past in childbirth?

Death in childbirth. They die due to worsening of medical problems or due to pregnancy associated issues both now as well as in the past.
Infection. 100 years ago, 1% (1 in100) women died from chiildbirth, usually from infection, bleeding, or failure to progress in labor (baby did not fit through the pelvis). That means that if you had 10 children in your lifetime, you had a 10% of death from chilbirth. Today 1 in 10000 women die from childbirth, leading cause of death being hemorrhage (bleeding), so things have improved.