Sixteen studies (randomized controlled trials) carried out in several different countries, consistently found that having continuous doula support during childbirth has a positive impact on labour and birth.
Some of these benefits include:
Enkin et al.- A Guide to Effective Care in Pregnancy and Childbirth
These results are so outstanding that that one researcher stated
If a doula were a drug, it would be unethical not to provide it.
Klaus, Kennell and Klaus - Mothering the Mother
...and what do other experts say?
The evidence for doulas comes from more than eleven carefully designed studies: Quite simply, hiring one cuts in half the odds of you having an unnecessary cesarean… the doula you bring with you may be the only person whose sole responsibility is to make you more comfortable and to help you labor as effectively as possible.
Ina May Gaskin - Ina May’s Guide to Childbirth
Though a friend can certainly be a labor support person, mothers typically have the best results when they hire a professional labor assistant (…also called a labor support doula…) who in addition to providing comfort and companionship to the laboring mother, has special obstetrical training…Her knowledge of and experience with birthing, and her sole focus on the mother’s needs, make her a unique, and to our minds, an indispensable part of the hospital birth
Dr. William Sears -The Pregnancy Book
You needn’t worry that a doula will replace your partner. A good doula enhances rather than detracts from the father’s participation…Interviewed afterward, fathers uniformly liked having a doula and did not feel that she interfered with their role. Fathers reported that not only did doulas help them help the moms, but many of the doula’s support actions were directed personally at them… Doulas improve physiological and social outcomes.
Henci Goer -The Thinking Woman’s Guide to a Better Birth