There are two main theories as to why doulas are so effective: the "Harsh Environment Theory" and because doulas are a form of pain relief (no really! here is the study: Hofmeyr, 1991)
The "Harsh Environment Theory" basically says that a doula buffers the harsh environment that sometimes comes with birth, and helps a woman stay calm and focused on her labor instead of the distractions of her environment.
Doulas are called a "Pain Relief" because of the decreased use of pain relief with labor. In my opinion, this is because doulas help mothers stay out of the Fear > Tension > Pain loop. Human nature causes our muscles to tense when we are afraid, and with tension we feel more pain. With more pain, we feel more fear. With more fear, we feel more tension that leads to more pain. This goes on and on and on, and as a woman that has been in that loop, I can testify to how hard it is to get out of it. I hold many purposeful conversations and activities with clients from the moment I am hired, to when a client first starts labor, until the last moments that the mother is pushing out her baby to help keep her emotionally and mentally calm, and feeling safe. This is so important. It is what I would define as the single most valuable benefit to having a doula.
(content credit: Hofmeyr, Nikodern et al 1991, evidence based birth)
Katie Mathis Birth Photography
I provide emotional support. I have a tool box of emotional and physical comfort measures including: breathing, relaxation, movement and positioning exercises. I help provide information and resources to assist in decision making,. I continuously reassure and comfort the laboring mother, to reminder her that she is strong, she can do this and this will not last forever. I help parents feel informed and empowered after their birth experience, but encouraging them to stay involved in the decisions for their care.
Well, I'm glad you asked. Labors that involve the continuous support of a doula can brag the following stats:
31% decrease in Pitocin
28% decrease in the risk of cesarean birth
12% increase in the likelihood of spontaneous vaginal birth
94% decrease in the use of any medications for pain relief
14% decrease in the risk of newborns being admitted to a special care nursery
34% decrease in the risk of being dissatisfied with the birth experience