I was told I was having a girl at my 16week ultrasound I'm almost 31weeks now and she's still a girl! As long as baby is in a good position and you have a great ultrasound tech they should be able to tell you no problem, good luck :)
Yupp both times I had it done at 17 weeks also & they were spot on.. but it could all depend on what position baby is in to be able to get a good look.