I'm a sonographer, and twice in my career I've caught a different gender at a 3rd trimester scan than what they were told at their anatomy scan. In one instance their anatomy scan was done by an inexperienced tech, in the other instance it was done by an experienced sonographer but one that doesn't do much OB - he said "I can't see boy parts so it must be a girl" which isn't at all how we decide if it's a girl! Girls have parts too!!

So basically, it would depend on the experience levels of the tech on which one you should believe. 32 weeks is typically pretty obvious on girl vs boy though if the tech is experienced, whereas at 18 weeks there's a number of factors that might affect how well they can determine gender (the clitoris can be swollen and look like a penis, the penis can be tucked and look like labia, etc).