What is a normal birth?

What is a natural birth?

What is an optimal birth?

All 3 questions are asking different things, and the answers will be different to each question.

'Normal' is largely defined by culture and time. C sections or assisted vaginal birth are certainly 'normal' in Australia at this time.
e.g. it used to be accepted (i think in the 60s) that a birth lasting 48 hours was 'normal'. In the 70s, normal was redefined as a birth lasting up to 24 hours. Now, a labour over 12 hours is considered long and not 'normal'. (this last step was due to a Scottish dr at a very busy overcrowded hospital describing the 1cm dilation every hour rule to define normal).

And then there is 'normal physiological birth'...

I think 'natural birth' can be defined more concretely, but others may not agree. Not every woman wants a natural birth, even given the best circumstance not every woman will be able to safely achieve a natural birth.

Optimal birth is probably the most subjective term, what is optimal for me may not be optimal for you. What is optimal for the woman may not be optimal for society. What is optimal for me for this birth, may not be optimal for me next time.