I wouldn't stress so much about the technicality of it if you do need it. I think most of us would love to have a normal physiological childbirth, but it's not always possible, especially with things in the current state they are - hospitals today are not set up to give you the best chances possible.

A natural birth is one which involves no outside help in terms of pain relief, assisted delivery (forceps, vacuum etc) - but to have only gas or something like that is low intervention, but just not classed as 'natural'. For some gas is the only thing they need to distract them to avoid other interventions. For some it's not an option because they know it does circulate to the baby.

It's not the be all and end all, I think the most important thing to focus on is getting yourself informed and supported so you can best prepare for all this. 'New Active Birth' by Janet Balaskas is great and covers each type of pain relief well, so perhaps this might be a good book for you to grab. Another way to help with reducing the likelihood of pain relief if to get a Birth Attendant or Doula (click the link in my signature to find out the stats), and other things, but I see those as the main two. Get informed as best possible and get some trained, continuous support to help you work towards your birth intentions. Your Ob might agree with your intentions, but she wont be there until it's time to catch the baby (if she makes it) and you never know what midwife you will get and her philosopy, which will shift change around 2-3 times during a birth.