i will find soem for you, i have lots of research papers here form my midwifery training. if you're searching online you want to visit sites such as midirs (tink you might have to pay though for full article), british journal midwifery, look on hospital websites as their policies have articles/research used as evidence and you should be able to view them as public.
I think the cascade of intervention starts earlier -- when they tell you to lie down! Then the gas and air because it's more painful, then the pethidine to try something different, then the waters broken for you as it's all so slow, and so on and so forth to the epidural and the spinal block and the failed ventouse and the section. Sorry for the interruption, I have no stats. Only ranting opinion.
as far as i understand it, epiural pretty much leaves you immobile on the bed, you have to have more monitoring and a drip..that further restricts you. if the block is very dense you won't feel your contractions at all, and when it wears off for pushing, it can suddenly be extremely intense . if it is topped up before pushing, it can be very difficutl to push without feeling the urge and the contractions
simply not moving in labour and not using gravity to help move the baby down and not using positions that help the pelvis open up can slow descent
also, an epidural can relax the pelvic floor and perineum and stop the baby's head rotating correctly for the birth and that can increase the need for an instrumental delivery
epidurals can be really helpful if labour is long and tiring , if a mother has high BP it can reduce it, and it is the only way of really blocking the pain..
'Course it does depend what type of epidural. I had a mobile one and wasn't on my back at any point during labour. Laboured standing up and moving around and eventually gave birth (with no further intervention) in squatting position.
According to my obstetrician, epidurals can increase the likelihood of intervention only if they are administered too early before labour is fully established i.e. before you are 3cm dilated. Apparently once labour fully established, prospective studies show that rates of intervention are same as for vaginal births with no pain relief. I had a mobile epidural at around 3-4cm for my DS, had about 6 hours of labour total (this was my first baby!), DS came out after 20 mins of pushing, no episiotomy (had a couple of small stitches). Could walk with the epidural (walked to the loo), had a top-up when I was fully dilated and could feel all my contractions when pushing and push with them, just didn't feel any pain. Was completely relaxed through the whole process, read magazines in the delivery room while dilating to 10cm and chatting to the midwife! Gave birth on my side not my back to make exit a bit easier. Would have had to be strapped to monitor anyway even without pain relief as my waters had broken the day before and baby had to be monitored for any signs of distress. And had my baby brought to my breast immediately after birth, he wasn't sleepy and latched on without a problem. Think epidurals get a lot of bad press, I am very glad I had one as ended up having a very relaxing birth and DS came out very calm and happy with a perfectly round head!