I don't think blood work can be wrong on gender. Because I mean it's blood work!! 

My blood work was correct. We're having a boy. One of my friends at work told me her blood work was wrong. She was told it was a girl and her US showed a boy and she had a boy!

I'm worried about this too. My blood test said girl, but I keep thinking it's a boy. And now that I was told girl, I really hope it stays a girl! I expressed my concern with my OB at my 15w scan, so she did a between the legs scan and said it looked like girl there too, but we would know 100% certainty at my anatomy scan in 3 weeks. She also said she has never seen the blood test be wrong in the thousands she has ordered, so that was comforting. However, I know of one person whose blood test gave her the wrong gender, but it ended up being a lab error where they had mixed up some blood samples. The blood test is supposed to be 99.3% accurate, so there is a very very very slight chance it's wrong but highly unlikely.