Findings from recent Canadian studies on the knowledge and beliefs about birth practices among first-time pregnant women and among obstetricians and other birth providers indicate that many women are inadequately informed and many providers deliver non-evidence-based maternity care.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3210636/
I understand that the study was done in Canada, but I assure you if you read through you'll find that it's applicable in the US, as well.