Medical student and amateur historian here. This question is exceedingly difficult to answer because there are so many reasons in modern medicine for why a women would get a c-section. Also, defining "modern medicine" as middle ages and before is a stretch. At the earliest I would place the cutoff for modern medicine to be in the early 1900's but generally I like the use the advent of penicillin (1942) as the cutoff but I digress.

Some complications during pregnancy that call for a c-section would surely have an extremely high mortality for both the mother and child. For example a complete placental previa is when the placenta completely covers the internal cervical os. This means the placenta is covering the exit site from which the baby would be born. When the mother goes into labor the placenta can tear causing massive blood loss for both the baby and mother. With there being no imaging techniques, blood transfusions, or ways to stop the bleeding while the birth is occurring this surely wuld be a tragedy.

This is just on extreme example of a complication that when it occurred in the middle ages surely had a near 100% mortality for the child and very high mortality for the mother. But as I mentioned prior there are numerous reasons for a c-section so I can't lump everything together into one answer.

Hope this helps somewhat.