A certain amount would have been due to the young age of mothers, a body being physically too small to give birth makes labour far more complicated. However in most instances girls would marry after puberty set in and should be physically prepared for labour.

Many deaths would have been caused by poor hygiene and poor medical knowledge. When Mary I of England was 'pregnant' it was believed that a boy would be born after 9 months and a girl after 10 months. Medical forceps were not invented until 1620, but as there was little knowledge of germs and infection these often did more harm than good; doctors were often reluctant to use them due to the associated risks. Princess Charlotte was left for days before forceps were used and eventually bled to death. Problems with infection, bleeding and pain contributed massively to maternal and infant mortality.

I visit many of my county cemeteries doing restoration and Civil War research. Granted people didn't arrive into our area until the 1830s (Indiana) but most of the pregnancy deaths I have personally seen seem to vary in age. I don't recall seeing any under 18, and most are in their mid-twenties.

A local example would be the Ray family in Memorial Cemetery (scroll down toward the bottom). Josphine was the 1st wife of Edgar and died at 23. His second was was younger but she died at 19.