A woman who is supported by a doula during her pregnancy, labor and birth:
reduces her chance of a Cesarean delivery by 50%.
reduces her length of labor by 25%.
reduces her need for pitocin by 40%.
reduces her need for pain medication by 30%.
reduces her request for an epidural by 60%.
reduces her need for forceps assistance by 40%.
Additionally, other studies have shown that women attended by doulas experience fewer secondary complications-- maternal infection, maternal fever, newborn problems, postpartum depression (what people call "the baby blues"), to name a few.
Here is another source for statistics: Child Birth Connection