Gender reveal

Due to being 35 this year and being diagnosed with complete previa I am going to go through with the blood test and ultra sound to check for abnormalities. I am telling the tech and everyone there to keep the gender under wraps. I want to wait until the 20week scan and let them put the gender on a slip of paper in an envelope and give it to my sister who is throwing us a gender reveal party! I'm so nervous though because I want to know so bad but we really want to be surprised with everyone else. Anyone else do this, did anyone at the doctors office spill the beans to you??