Fatmama, as well as research that I had undertaken on my own whilst pregnant, the information I received was provided by the obstetric department at the hospital that I birthed at. It provided information on all of the available analgesia and included detailed information on the benefits and risks of each type of pain relief, including (amongst other things) the increased likelihood for reduced contractions or stalled labour requiring oxytocin supplementation and an inability to push as a result of loss of sensation due to epidural, which can result in instrumental delivery or caesarean section. It is not just a myth perpetuated by the 'midwifery lobby'. Much of the research to support this is performed by obstetricians and published in obstetrics journals.

To answer your question about bias, some obstetricians support a medicalised approach to birth because they are, after all, trained doctors, and their methodology is shaped by medical training. They can see birth as a process requiring pain-relief because pain management is something that doctors are trained in - the see pain as a symptom of a problem and seek to avoid it where possible. They are also specialists who are trained to manage the problems and complications associated with pregnancy and births, so in some ways they are like a nuclear physicist at a barbecue - at an uncomplicated birth they may apply their specialist training to an extent that is not necessary. Whereas most midwives (and some obstetricians) encourage as little intervention as possible because it has been demonstrated that in an uncomplicated birth, the outcome for both mother and child is better. Funnily enough, the main agenda of either faction is to ensure the best outcome for birthing women, yet a much better outcome can be achieved when these stakeholders see their roles as complementary rather than oppositional. Disregard for the benefits of either role can be detrimental to outcomes and to choice.

Certainly there are people who do benefit from an epidural (and FWIW, I was one of them, although I had hoped to birth without intervention I was one of those who did benefit from it), but this does not negate that there are risks to both mother and child.