At your 20week anomally scan they can get it wrong,and be only about 80% sure of the sex of a baby. At the 4d scan they are more likely to get it right as you can see things more clearly, but still its not 100%. They can and do get it wrong, I have seen a lady have a baby girl and she was told it was going to be a boy. All she had was blue stuff for the baby, so i would always think to myself, right they say its a girl, ill buy just a few pink things and the rest can be white until the baby is born. xx