"I think if a person has a preference for want of a better word then it's better to find out so that you have 20 weeks to get used to the idea."
I don't agree paddyclamp - at least, that would not have worked for me at all. When I was pg with dc3, after two girls, I was worried about how I would feel if I had a boy. I felt I understood girls, but boys were a "foreign country". I didn't find out the sex and I had a ds. I was amazed that he was a boy, but amazed in a good way. I honestly think that if I had known he was a boy at the 20 weeks scan I would have spent 20 weeks worrying about whether I was going to bond with this child from a foreign country.
I also think that, when you find out the sex at a scan, that is ALL you find out. Unlike at the birth, you don't find out how they smell, what they look like, how they feel, how they sound ... it becomes the ONLY thing you know about that child. And not only that, but you find out what it is not - ie that it is not a boy. And if that takes you out of your comfort zone, there's nothing much else to fall back on ("but she is so beautiful/such tiny fingers/smells so gorgeous" etc). The only info you have is "negative" info - she is not a boy.
Once you have your baby, Winky, she will be that first - your baby. Feeding her, washing her, taking care of her - all these things are unaffected by the fact that she is a girl (apart from the obvious difference under the nappy). By the time she starts exhibiting characteristics that set her apart as a girl - which probably won't happen until she is about two! - she will be so much your child and you will be so used to her being yours, that it simply won't figure as an issue.
When I had ds I regularly used to forget he was a boy at first - I would undo his nappy to change him and get a shock because I kept forgetting about the extra tackle! But the reason I kept forgetting was because it was unimportant in the scheme of things - he was my baby and I loved him for that.
I hope my incoherent ramblings have helped in some way.