Helping Labor/Delivery...

I have been reading these threads everyday! So very helpful.
My question for today is....do MOST births go on without help from us humans? I have read about cats not 'cutting the cord' etc and am wondering if this and other things like this, happen often.
And how do we know when to step in and help? Will it be OBVIOUS that she might need help? Also, should we take the first kitten(s) out if she is still in labor and then put them back, or just leave them with her????
Of course I will her if she needs help but I have to be honest, I would rather stay out of it!!

Looking back over the past 2-1/2 years, I have had 3 litters born here, totalling 15 kittens. I helped revive one kitten. I do not know if the Mom would have cleaned it up if I had ignored it, but since she seemed to be ignoring it, still in the bag, I took over. Otherwise, I am very hands off with my deliveries.

Of course, I had only had each of my Momcats for a matter of days. While they were all very nice kitties, I did not want to disturb them. I think in general the Momma does all the work, and we are there as a back up, just in case. So prepare for the worst and hope for the best!