Im sorry i cant help as I am hopeless at knowing what is what on scans. But i have heard that if they think it is a girl at first then it is not certain as sometimes it can grow at different rates and they might not be able to get the best look at teh time of the scan. If they tell you it is a boy then they are normally right. They can make mistakes but if the bits are there - then the bits are there! IYKWIM!

I've actually heard that too. They can't say its a boy unless they actually see the bits. So it often happens that you are told girl & get boy, but its pretty rare the other way around. Usually if they say boy, its a boy.
Maybe you could ask for another scan. Just tell your gp you need to be sure to organise yourself.
A friend of mine went back twice & had an appt booked each following week just to find out the sex of her DD.