I would say girl but I am not convinced it's possible to tell until you have at least one sibling to compare with.
When I posted the ultrasound picture everyone said boy, until I posted a comparison with my daughters ultrasound, where the heads are identical!
My boys had very rounded heads with sloping foreheads and my daughter had a very clear straight forehead, so for now I am inclined to thing a straight forehead is more likely to be a girl. But that's just from my own experience, and my bump could well be blue, so the whole theory could go out the window