How reliable is an ultrasound after 18 weeks?

So I had my ultrasound done at 19+2 and I was planning on waiting and having a surprise baby. During the ultrasound I was asked to close my eyes at certain times so I wouldn't see what it could be. I had them put the results in a sealed envelope so I could give to my husband on his birthday which is also Christmas eve. Long story short the tech ended up accidentally saying "he" at the end when we couldn't get the baby to move to get a better picture of its heart. My husband ended up opening the envelope to confirm that it was indeed a boy. We're very happy but now people have said that it could still be wrong and since I didn't actually see what it was I'm worried they could be right. Do you think I can go by what the envelope said or should I have another ultrasound scheduled????