The second wife of Charlemagne comes to mind. She died at around 25 due to child birth related issues. While a bit out of the timeframe,
Jane Seymour third wife of King Henry VIII died at ~28. Also
Eleanor de Montfort and
Eleanor de Braose both being in their mid 20s. Then there's
Judith of Habsburg and
Isabella of Hainault. These are just off the top of my head anyways, as I know there are many, many more examples
Being a peasant or a noble mattered relatively little when it came to child birth during that era as it was a common practice to have a midwife deliver the baby, not a physician. Even if a physician were to deliver the baby, physicians knew laughably little and what they did "know" was often a load of hogwash and snake oils. Chances are the midwife actually would have known better anyways.
Assuming the lady in question survived the birth itself, a task that sometimes even proved too much on its own, they could then start worrying about post partum infections and the like. Being a pregnant woman during the feudal period quite simply sucked.
EDIT: Although this does extend in parts to after CK2's timeframe as well,
here's a list of some notable female deaths during childbirth.
And these are the women "we care about" to put it bluntly. I wouldn't be surprised if there are many women who were never noted into chronicles as well.
