I did it 3 times (giving birth, that is) and they were all over 8lbs. I had them very quickly, my longest labour being the first - 4 hours. I found contractions painful but the actual pushing them out just hard work.
I had studied and followed a book on natural childbirth by Dr. Grantly Dick-Read. His methods were based on yoga, fitness and relaxation and I am quite a titch, only 5'1 and small-boned, but I did all the exercises and learned to relax my whole body "at will". It worked for me.
Having said that, with my 2 daughters I had spent 9 months in a perpetual state of vomitting. Morning sickness? it was all bl**dy day and night sickness - and a craving for draft bitter with the first one (peaches and Lucozade with daughter no. 2). Only my son behaved himself while he was brewing - and he made up for it 15 years later.
So why do women keep on doing it? Mother Nature has a very firm hand and the urge to reproduce is, for those who experience it, something you can't always resist. No matter how awful the experience of birth there's nothing like the feeling when you hold your beautiful sleeping baby in your arms, at peace with the world. We'll skip over the feeling when it's screaming its head off at 3 a.m. as you change a nappy full of ****.
