Oh you poor thing it sux doesnt it??
We were told at 20 wks that DD was a girl by two different scans and then at 35 weeks that "hmmmm nope theres something there more likely to be a boy!!"

We went for a 3D scan and turns out it was her hand down there that he saw but it still was enough to put that doubt in my head and confuse me.

Your 20 wk scan doesnt seem that conclusive though but if you have already prepared for another little lady then I would stick with it.
Although in saying that the beautiful ladies at the 3D clinic were so wonderful and said that it was "hugely" obvious at that late stage if it is a little boy.

Good luck