I'm sorry for plugging my blog, but I've been thinking about this a lot recently from the other end of the equation (as a new antenatal teacher - not NCT, but with quite a few sympathies with them).
Funnily enough, I wrote this post last night about education without blame.
The difficulty is that if you teach women to have low expectations, it reduces their chance of getting the birth they want. If they go into labour with a good idea of what they want, they feel more able to support themselves and to ask for what they need. Getting a 'natural' birth if you want one has a lot to do with positive thinking.
But it doesn't always work, no matter how determined you are. And the difficulty for antenatal teachers is that if it doesn't and we've said 'natural birth is possible' that we're charged with creating unrealistic expectations and women have farther to fall into disappointment (and birth trauma and PND).
From personal experience, I had a caesarean with my first baby after I planned a home birth and then failed to progress. Subsequently, I had two VBACs.
I was disappointed with my caesarean - I'd been to NCT classes and thought natural birth was just a matter of effort. Following the C-Section, I felt that it was all a bit random. My first VBAC was relatively easy. My second one was slow and stalling and I felt I had to 'work hard' - keep very active, do a lot of mental visualisation.
There's no ideal answer. I feel as educators, we have to be empowering. We have to encourage women and show them that their body is capable of great things. We also have to show them how to get the support they need and to maximise their chances of getting the birth they want. But we have to also prepare them that sometimes it doesn't all go to plan, it's not their fault and be there for them afterwards (which is where a lot of antenatal teachers fail.)
Will be watching this thread with interest as I'm planning more work in this area.