There have been many studies done to show the link between a continuous support person and the rate of cesarean births. Studies show that not only do birth doulas lower the chance of cesarean birth, they also lower the rate of epidurals, forceps, vacuums, and other medical interventions. As a result, doulas can increase the immediacy of breastfeeding, and bonding for the family.
The advantage to having a doula, especially if this is your first child, is that the doula has been the support in labour before, and has been trained to support the couple in labour. (Keep in mind that every labour is different, and even if you are pregnant with your third child, you can still benefit from having a doula) Many times, the labouring woman and her partner are in the room alone, and unless the partner has read the books and remembered everything from the prenatal classes, they may be at a loss as to what to do. Doulas can suggest different possitions, can reassure the woman that she is doing a great job (and that the partner is doing well too), can get water for the woman so that she is never alone in the room, and so much more.