While I can't point you in the direction of research, I work with a lactation consultant and I know her experience is that epidurals can interfere with breastfeeding success. Baby will be in varying states of grogginess depending on how long the epidural was in. This can impact baby's ability to coordinate his/her sucking properly. Interestingly, she's never been unable to get a baby from a drug-free birth to latch on. There are many mom's that had epidurals that are able to nurse successfully, but it seems to add a wrinkle to the process. And with a mom that's already feeling overwhelmed, that may be enough to discourage her from breastfeeding.
Also, epidurals can increase the need for other interventions being used such as vacuum or forceps and that can most certainly cause problems such as head pain, tearing in the membranes that surround the brain, and nerve damage. I think that would really be my biggest reason to avoid an epidural.
There are other times though when an epidural can be what the mom needs to relax and help the birth progress. So, while I prefer the idea of an unmedicated birth, there are lots of different roads to a successful birth and that's really an individual process.
Hope this helps!
Lisa , married to Dan, mama to IVF miracle Natalie 5/20/09 :