Well, the first place that I went to was for a level 2 u/s as dh had cleft palette as a baby so gender determination was not a priority. (where I live, they do not tell gender)
So they were not focused completely on determining sex. I just think at the end they may have taken a half hearted look and did not see anything so figured girl. I think (now I know) the angle was just not right to tell 100%.
You could still likely get in to a teaching school that does u/s's or something along those lines if you are still iffy. We always hear of the people or people that know people that were told one sex and had another. Just never thought I would be one of them.
With my DS#2, I saw immediately that he was all boy. Tech was just moving the wand and there he was, in all his glory.