Don't go off the H/B at all, I just went through the SAME thing, found out at 18 weeks that she was a she, and was worried up until now (35w1d) and I had an U/S last week to confirm it's DEFINITELY a girl. It's pretty hard for a boy to hide his stuff, so I'd try to have some confidence in her guess!
I found out at 20 weeks I was having a girl but they didnt seem too sure so I had a 3d/4d ultrasound done and they said girl and I still wasnt sure if I believed them lol but at my growth scan at 36 weeks they said it was 100% a girl and I could definitely tell it was this time.
I live in the UK and we only get 2 scans here :( I just am really scared they are wrong. I'm all prepared for a girl, from her clothes to her room.
They should do another sono around 32 weeks to check the baby's percentile. Ask the tech to take a look at the genatals again
It has happened before. Its usually pretty rare though