A doula is the only birth intervention that has absolutely no negative side effects. In fact, the only effect a doula or labor support can have on birth is a positive one.

Here is a collection of medical studies published over the last 30 years regarding doula care. The studies are listed in order from latest to earliest. Enjoy!
A randomized controlled trial of continuous labor support for middle-class couples: effect on cesarean delivery rates
McGrath SK, Kennell JH. Birth. 2008 Jun;35(2):92-7.
http://www.ncbi.nlm.nih.gov/pubmed/18507579
The doula group had a significantly lower cesarean delivery rate than the control group (13.4% vs 25.0%), and fewer women in the doula group received epidural analgesia (64.7% vs 76.0%). Among women with induced labor, those supported by a doula had a much lower rate of cesarean delivery than those in the control group (12.5% vs 58.8%). On questionnaires the day after delivery, 100 percent of couples with doula support rated their experience with the doula positively.
For middle-class women laboring with the support of their male partner, the continuous presence of a doula during labor significantly decreased the likelihood of cesarean delivery and reduced the need for epidural analgesia. Women and their male partners were unequivocal in their positive opinions about laboring with the support of a doula.

Female relatives or friends trained as labor doulas: outcomes at 6 to 8 weeks postpartum 
Campbell D, Scott KD, Klaus MH, Falk M.
Birth. 2007 Sep;34(3):220-7.
http://www.ncbi.nlm.nih.gov/pubmed/17718872
Six hundred low-risk, nulliparous women were enrolled in the original clinical trial and randomized to doula support (n = 300) or standard care (n = 300). The mother-to-be and her doula attended two 2-hour classes about providing nonmedical, continuous support to laboring women. For the secondary study, presented here, research participants (N = 494) were interviewed by telephone using a 42-item questionnaire.
Overall, when doula-supported mothers (n = 229) were compared with mothers who received standard care (n = 265), they were more likely to report positive prenatal expectations about childbirth and positive perceptions of their infants, support from others, and self-worth. Doula-supported mothers were also most likely to have breastfed and to have been very satisfied with the care they received at the hospital.
Labor support by a minimally trained female friend or relative, selected by the mother-to-be, enhances the postpartum well-being of nulliparous mothers and their infants, and is a low-cost alternative to professional doulas.

A Randomized Control Trial of Continuous Support in Labor by a Lay Doula 
Della A. Campbell, Marian F. Lake, Michele Falk, and Jeffrey R. Backstrand JOGNN 2006, AWHONN, the Association of Women’s Health, Obstetric and Neonatal Nurses
http://newark-rutgers.academia.edu/JeffreyBackstrand/Papers/712571/A_randomized_control_trial_of_continuous_support_in_labor_by_a_lay_doula
Significantly shorter length of labor in the doula group, greater cervical dilation at the time of epidural anesthesia, and higher Apgar scores at both 1 and 5 minutes. Differences did not reach statistical significance in type of analge-sia/anesthesia or cesarean delivery despite a trend toward lower cesarean delivery rates in the doula group.
Providing low-income pregnant women with the option to choose a female friend who has received lay doula training and will act as doula during labor, along with other family members, shortens the labor process.

Caregiver support for postpartum depression
Ray KL, Hodnett ED.
Cochrane Database Syst Rev. 2001;(3):CD000946.
http://www.ncbi.nlm.nih.gov/pubmed/11686971
Treatment of postpartum depression with support was associated with a reduction in depression at 25 weeks after giving birth (66% less). There is some indication that professional and/or social support may help in the treatment of postpartum depression.

Caregiver support for women during childbirth 
Hodnett ED.
Cochrane Database Syst Rev 2000;2:CD000199. http://www.childbirthconnection.org/pdfs/continuous_support.pdf
Fourteen trials, involving more than 5,000 women, are included in the review. The continuous presence of a support person;
-  reduced the likelihood of medication for pain relief (29% less)
-  reduced the likelihood of operative vaginal delivery (23% less)
-  reduced the likelihood of cesarean delivery (23% less)
-  reduced the likelihood of five-minute Apgar score of less than 7 (50% less)
Continuous support also was associated with a slight reduction in the length of labor. Six trials evaluated the effect of support on mothers’ views of their childbirth experience; although the trials used different measures (overall satisfaction, failure to cope well during labor, finding labor to be worse than expected, and the level of personal control during childbirth), in each trial the results favored the group that had received continuous support.
Continuous support from professional health care workers or nonprofessional caregivers during labor and delivery has a number of medical and psychosocial benefits for mothers and their babies, and there does not appear to be any harmful effect.

Alternative Strategy to Decrease Cesarean Section: Support by Doulas During Labor 
Guadalupe Trueba, LCCE, FACCE, CD (DONA), Midwife, Carlos Contreras, MD, Maria Teresa Velazco, MD, Enrique García Lara, MD, and Hugo B. Martínez, MD
J Perinat Educ. 2000 Spring; 9(2): 8–13. PMCID: PMC1595013 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1595013/
This research was conducted in a public general hospital in Mexico City, Mexico. The objective was to evaluate efficacy of the support given by a doula during labor to reduce cesarean rate. From March 1997 to February 1998, a group of 100 pregnant women were studied. These women were at term, engaged in an active phase of labor, exhibited 3 cm. or more cervical dilatation, were nuliparous, had no previous uterine incision, and possessed adequate pelvises. The group was randomly divided into two subgroups comprising 50 women, each: The first subgroup had the support of a childbirth educator trained as a doula, while the second subgroup did not have doula support. Measurements were recorded on the duration of labor, the use of pitocin, and whether or not the birth was a vaginal birth or cesarean section. Characteristics and gestational age were similar in both groups. Results confirmed that support by doulas during labor was associated with a significant reduction in cesarean birth and pitocin administration. There was a trend toward shorter labors and less use of epidurals. The results of this study showed, as in other trials measuring the impact of a doula’s presence during labor and birth, that doula support during labor is associated with positive outcomes that have physical, emotional, and economic implications.

Maternal Social Support Predicts Birth Weight and Fetal Growth in Human Pregnancy
PAMELA J. FELDMAN, PHD, CHRISTINE DUNKEL-SCHETTER, PHD, CURT A. SANDMAN, PHD, AND PATHIK D. WADHWA, MD, PHD
Psychosomatic Medicine 62:715–725 (2000) 715, 0033-3174/00/6205-0715 2000
by the American Psychosomatic Society http://health.psych.ucla.edu/CDS/pubs/2000%20FeldmanDunkelSchetter_Maternal%20social%20support.pdf
Objective: Low birth weight is a primary cause of infant mortality and morbidity. Results of previous studies suggest that social support may be related to higher birth weight through fetal growth processes, although the findings have been inconsistent. The purpose of this investigation was to test a model of the association between a latent prenatal social support factor and fetal growth while taking into account relations between sociodemographic and obstetric risk factors and birth weight.
Method: A prospective study was conducted among 247 women with a singleton, intrauterine pregnancy receiving care in two university-affiliated prenatal clinics. Measures of support included support from family, support from the baby’s father, and general functional support. Sociodemographic characteristics were also assessed. Birth outcome and obstetric risk information were abstracted from patients’ medical charts after delivery.
Results: Structural equation modeling analyses showed that a latent social support factor significantly predicted fetal growth (birth weight adjusted for length of gestation) with infant sex, obstetric risk, and ethnicity in the model. Marital status and education were indirectly related to fetal growth through social support. The final model with social support and other variables accounted for 31% of the variance in fetal growth. 
Conclusions: These findings suggest that prenatal social support is associated with infant birth weight through processes involving fetal growth rather than those involving timing of delivery. Biological and behavioral factors may contribute to the association between support and fetal growth, although these mechanisms need to be further explored. These results pave the way for additional research on fetal growth mechanisms and provide a basis for support intervention research.

Effects of providing hospital-based doulas in health maintenance organization hospitals
Gordon NP, Walton D, McAdam E, et al.Obstet Gynecol 1999;93:422–6. http://journals.lww.com/greenjournal/Fulltext/1999/03000/Effects_of_Providing_Hospital_Based_Doulas_in.22.aspx
In general, women who had doulas were very enthusiastic about them, and even women who did not have doulas thought they would like to have one if they had another baby. The women appreciated the knowledge doulas had of labor, the support and reassurance doulas provided, and because there was an extra person to relieve the spouse-partner, they were never alone. As hospitals increase the ratio of patients to labor and delivery nurses, consequently, they decrease the amount of direct contact and support of women in labor by those nurses. Over the years, technology has replaced the human aspect of patient care without any improvement of outcomes, and it might be appropriate to reassess the effect of experienced women labor companions on labor and delivery.

A comparison of intermittent and continuous support during labor: a meta-analysis
Scott KD, Berkowitz G, Klaus M. Am J Obstet Gynecol. 1999 May;180(5):1054-9. http://www.ncbi.nlm.nih.gov/pubmed/10329855
Our goal was to contrast the influence of intermittent and continuous support provided by doulas during labor and delivery on 5 childbirth outcomes. Data were aggregated across 11 clinical trials by means of meta-analytic techniques.
Continuous support, when compared with no doula support, was significantly associated with:
- shorter labors (weighted mean difference -1.64 hours)
- decreased need for the use of any analgesia (36% less)
- oxytocin (71% less)
- forceps (57% less)
- cesarean sections (51% less)
Intermittent support was not significantly associated with any of the outcomes. Odds ratios differed between the 2 groups of studies for each outcome. Continuous support appears to have a greater beneficial impact on the 5 outcomes than intermittent support.

The obstetrical and postpartum benefits of continuous support during childbirth
Scott KD, Klaus PH, Klaus MH.
J Womens Health Gend Based Med. 1999 Dec;8(10):1257-64. http://www.ncbi.nlm.nih.gov/pubmed/10643833
The purpose of this article is to review the evidence regarding the effectiveness of continuous support provided by a trained laywoman (doula) during childbirth on obstetrical and postpartum outcomes. Twelve individual randomized trials have compared obstetrical and postpartum outcomes between doula-supported women and women who did not receive doula support during childbirth.
Three meta-analyses, which used different approaches, have been performed on the results of the clinical trials. Emotional and physical support significantly shortens labor and decreases the need for cesarean deliveries, forceps and vacuum extraction, oxytocin augmentation, and analgesia. Doula-supported mothers also rate childbirth as less difficult and painful than do women not supported by a doula. Labor support by fathers does not appear to produce similar obstetrical benefits. 
Eight of the 12 trials report early or late psychosocial benefits of doula support. Early benefits include reductions in state anxiety scores, positive feelings about the birth experience, and increased rates of breastfeeding initiation. 
Later postpartum benefits include decreased symptoms of depression, improved self-esteem, exclusive breastfeeding, and increased sensitivity of the mother to her child’s needs. The results of these 12 trials strongly suggest that doula support is an essential component of childbirth. A thorough reorganization of current birth practices is in order to ensure that every woman has access to continuous emotional and physical support during labor.

Effects of psychosocial support during labour and childbirth on breastfeeding, medical interventions, and mothers’ wellbeing in a Mexican public hospital: a randomised clinical trial 
Ana Langer et al
British Journal of Obstetrics and Gynaecology, October 1998, Bol 105, pp. 1056-1063 http://www.cebp.nl/media/m498.pdf
The frequency of exclusive breastfeeding one month after birth was significantly higher (64% higher) in the intervention group, as were the behaviours that promote breastfeeding. More women in the intervention group perceived a high degree of control over the delivery esperience, and the duration of labour was shorter than in the control group (4.56 hours vs 5.58 hours). 
Psychosocial support by doulas had a positive effect on breastfeeding and duration of labour. It is important to include psychosocial support as a component of breastfeeding promotion strategies.

The effects of doula support during labor on mother-infant interaction at 2 months
Landry SH, McGrath SK, Kennell JH, et al. Pediatr Res 1998;43:13A. http://www.nature.com/pr/journal/v43/n4s/full/pr1998210a.html
The M-I interaction scores revealed significant differences between the doula-supported and no-doula group of mothers. The doula supported mothers’ mean interaction score was significantly higher than the no-doula group of mothers for 4 of 5 observation points. Overall, the mean M-I interaction scores of all 5 observation points for the doula and no-doula groups respectively were 5.61 and 4.75.
Overall, results demonstrate that providing a laboring woman with the continuous support of a doula results in a significantly more positive level of interaction with her infant at 2 months after delivery. Questions remain regarding the physiological and/or psychological mechanism that could explain this powerful and long-lasting effect of doula support.

Continuous labor support from labor attendant for primiparous women: a meta-analysis
Zhang J, Bernasko JW, Leybovich E, Fahs M, Hatch MC.
Obstet Gynecol. 1996 Oct;88(4 Pt 2):739-44.
http://www.ncbi.nlm.nih.gov/pubmed/8841285
Meta-analysis of four studies conducted among young, low-income, primiparous women who gave birth on a busy labor floor in the absence of a companion suggested that continuous labor support by a labor attendant shortens the duration of labor by 2.8 hours, doubles spontaneous vaginal birth, and halves the frequency of oxytocin use, forceps use, and cesarean delivery rate. Women with labor support also reported higher satisfaction and a better postpartum course. 
Labor support may have important positive effects on obstetric outcomes among young, disadvantaged women. 

Postpartum depression and companionship in the clinical birth environment: a randomized, controlled study
Wolman WL, Chalmers B, Hofmeyr GJ, Nikodem VC.
Am J Obstet Gynecol 1993;168:1388–93.
http://www.ncbi.nlm.nih.gov/pubmed/8498417
Postpartum depression is a common feature of childbearing and is the cause of considerable morbidity. We have explored the possibility that clinically oriented care during labor may contribute to its occurrence.
Of 189 nulliparous women laboring in a familiar community hospital, 92 were allocated by randomized, sealed envelopes to receive additional companionship from one of three volunteer labor companions recruited from the community.
The group receiving support attained higher self-esteem scores and lower postpartum depression and anxiety ratings 6 weeks after delivery. 
In the clinical labor environment companionship modifies factors that contribute to the development of postnatal depression. We emphasize the importance of paying attention to the psychosocial environment in which labor takes place, to facilitate adaptation to parenthood. 

A randomized controlled trial
Kennell J, Klaus M, McGrath S, Robertson S, Hinkley C.
Journal of American Medical Association. 1991;265(17):2197–2201.
http://jama.jamanetwork.com/article.aspx?articleid=385782
The continuous presence of a supportive companion (doula) during labor and delivery in two studies in Guatemala shortened labor and reduced the need for cesarean section and other interventions. In a US hospital with modern obstetric practices, 412 healthy nulliparous women in labor were randomly assigned to a supported group (n = 212) that received the continuous support of a doula or an observed group (n = 200) that was monitored by an inconspicuous observer. Two hundred four women were assigned to a control group after delivery. Continuous labor support significantly reduced the rate of cesarean section deliveries (supported group, 8%; observed group, 13%; and control group, 18%) and forceps deliveries. Epidural anesthesia for spontaneous vaginal deliveries varied across the three groups (supported group, 7.8%; observed group, 22.6%; and control group, 55.3%). Oxytocin use, duration of labor, prolonged infant hospitalization, and maternal fever followed a similar pattern. The beneficial effects of labor support underscore the need for a review of current obstetric practices. 

Companionship to modify the clinical birth environment: effects on progress and perceptions of labour, and breastfeeding 
Hofmeyr, G. Justus, Nikodem, V. Cheryl, Wolman, Wendy-Lynne, Chalmers, Beverley E., Kramer, Tami
British Journal of Obstetrics and Gynecology, ISSN: 0306-5456, 1991 http://www.readabstracts.com/Health/Companionship-to-modify-the-clinical-birth-environment-effects-on-progress-and-perceptions-of-labour.html#b#ixzz20g3J8I4J
Although women in labor were once isolated from friends and family, many hospitals now encourage companions, usually male partners, to be present during labor. However, one study showed that first-time fathers provided less support during labor than female labor companions. It was also shown that women provided with continuous support from a female companion during labor had shorter labors and fewer complications than women without female companionship. Social support during pregnancy was shown to improve psychosocial but not physiological factors.
In another study, support during childbirth was associated with less need for pain-relieving agents and episiotomy (surgical incision of the outer birth canal to prevent tearing of the tissue during delivery). However, support did not reduce the length of labor or rate of cesarean section. The effects of supportive companionship on labor and adaptation to parenthood were assessed. Support companions were community volunteers without a medical background who were asked to focus on providing pregnant women with comfort, reassurance, and praise.
Although supportive companionship did not affect the progress of labor, it reduced blood pressure and the need for pain-relieving agents during labor. Women with support companions during labor felt a greater ability to cope and had less pain and anxiety during labor than women without support companions. In addition, women with support companions during labor were more likely to be breast feeding exclusively at six weeks and to be feeding their infants at flexible intervals. These findings show that companionship during labor may help to promote the self-esteem of laboring women and reduce adverse effects of labor in a clinical environment.

The effects of social support on women’s health during pregnancy, labor and delivery, and the postpartum period
Gjerdingen DK, Froberg DG, Fontaine P. Fam Med. 1991 Jul;23(5):370-5.
http://www.ncbi.nlm.nih.gov/pubmed/1884933
During pregnancy, emotional and tangible support provided by the spouse and others is related to the expectant mother’s mental well-being. In addition, informational support in the form of prenatal classes is related to decreased maternal physical complications during labor and delivery, and to improved physical and mental health postpartum. Mothers who have the support of a companion during labor and delivery experience fewer childbirth complications and less postpartum depression. Mothers’ postpartum mental health is related to both the emotional support and practical help (eg, housework and child care activities) provided by the husband and others. Health care providers are in a unique position to educate prospective parents about the importance of social support around the time of childbirth and may play a critical role in mobilizing support systems for new mothers.

Companionship to modify the clinical birth environment: effects on progress and perceptions of labour, and breastfeeding
Hofmeyr GJ, Nikodem VC, Wolman WL, Chalmers BE, Kramer T.
Br J Obstet Gynaecol. 1991 Aug;98(8):756-64.
http://www.ncbi.nlm.nih.gov/pubmed/1911582
Results of those receiving companionship throughout labor:
- more likely to report that they felt that they had coped well during labour (60 vs 24%)
- less labour pain (26.0 vs 44.2) 
- less anxiety  (28.2 vs 37.8) 
- more likely to be breastfeeding exclusively at 6 weeks postpartum (51 vs 29%)
- more likely to be feeding at flexible intervals (81 vs 47%)
Labour in a clinical environment may undermine women’s feelings of competence, perceptions of labour, confidence in adapting to parenthood and initiation of successful breastfeeding. These effects may be reduced by the provision of additional companionship during labour aimed to promote self-esteem.

Effects of social support during parturition on maternal and infant morbidity
Klaus MH, Kennell JH, Robertson SS, Sosa R. BMJ. 1986;;293: 585-587.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1341377/?tool=pubmed
Because continuous social support during labour is a component of care in many societies but inconsistent in our own, the clinical effect of support during labour on maternal and neonatal morbidity were studied. Social support was provided by female companions. Four hundred and sixty five healthy primigravidous women were enrolled using a randomised design.
Compared with 249 women undergoing labour alone 168 women who had supportive female companions throughout labour had significantly fewer perinatal complications, including caesarean sections (7% v 17%) and oxytocin augmentation (2% v 13%), and fewer infants admitted to neonatal intensive care. Of the women who had an uncomplicated labour and delivery requiring no interventions, those with a companion had a significantly shorter duration of labour (7.7 hours v 15.5 hours). This study suggests that constant human support may be of great benefit to women during labour.