Labors tend to be shorter and with fewer complications with a birth doula present.  Women having a birth doula report having more positive feelings about their childbirth experience.  Doula assisted births have a reduced need for pitocin to induce labor or any other delivery assistance.  There is also a reduction for the need for pain medications, epidurals and c-section when a birth doula is present.
Statistically proven outcomes are:
31% decrease in the use of pitocin
28% decrease in the risk of c-section
12% increase in the likelihood of a spontaneous vaginal birth
9% decrease in the use of any medications for pain relief
14% decrease in the risk of newborns being admitted to a special care nusery
34% decrease in the risk of being dissatisfied with the birth experience