Can a 16 week sonogram to find out the be wrong?

i went yesterday to get a 16 week scan to find out the gender of my baby (for fun) and the tech said it was a girl. the reason why she said it was a girl was bc her legs were wide open and there was nothing floating in between lol BUT i didnt see the 3 lines your suppose to see. i mean w.e the is is fine (even though i alread have a baby girl) but hmmmm i mean they told me it was a girl but for some reason i feel like its not confirmed haha. Has anyone done an early scan and then found out at your 20 week that they told you the wrong ?