A doula is "a trained professional who provides continuous physical, emotional and informational support to a mother before, during and shortly after childbirth to help her achieve the healthiest, most satisfying experience possible." They don't provide any medical services but rather nurture, support, and guide families so they have better outcomes.
Evidence shows that having a doula present results in:
lower likelihood of needing Pitocin
reduced likelihood of having a cesarean birth
fewer requests for pain medications
fewer births needing forceps or vacuum extraction
more positive long-term memories
higher likelihood of rating their childbirth experience positively
better Apgar scores for babies
more self-esteem and confidence
improved postpartum mental health
In addition, partners feel supported during the experience which helps parents to have a better and more supportive environment during and after childbirth.