That you may change your ideas about what you want in labour while it is happening.
I thought it would be great to be in the big birthing pool in the big, comfortable room with some soothing music on the CD player but I would have rather been in a cupboard in the dark with no noise at all.
I think I was influenced by too much information on the 'ideal' birthing situation...
Also, when I previously thought about having a homebirth I decided against it because I thought it might be too intense having a midwife with me the whole time. Went to a birthing centre instead and was terrified every time she left the room!
I think everything I had was the opposite of my birthplan. Hospital, syntometrine, syntocin, forceps, vitamin K jab, doctor cutting the cord - come to think of it I didn't have pethidine. So be prepared for things to change.
DH was fantastic through the birth but he was exhausted by the end of it. Drove the 30 miles home, rang everyone he knew and collapsed. Didn't come to see me all day (phone switched off). Not great for a new (hormonal, emotional) mum who was convinced that he had died - why else would he not come back to see us on the best day of our lives?! So, try not to be too hard on partners - they've been through a lot too.
The most important thing I learnt though was that even though I thought I was going to die during labour having DS is the most fantastic experience ever. And I'm going to go through it all again in a couple of months!