How reliable is an ultrasound after 18 weeks?

Grissel • ;)
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 <a href="https://play.google.com/store/apps/details?id=com.glow.android.eve">Eve</a>. 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????