Gender blood test wrong???

Hey ladies... I went to an ultrasound place that also draws blood to do the gender test I did this at 10 weeks it came back a week and a half later as the gender being female... I took the results with me to my doctor's office who informed me there's a good chance these results could be WRONG!! I'm 17.5 weeks now but my Dr is waiting till I'm 20 weeks to do the anatomy ultrasound so we'll know for sure then! Any body else do a blood test and it was wrong??