Ultrasound mistake !

Hi guys ! How often do you think sonographers make mistakes at screening?  Last week I was at the screening for 20 weeks, she pushed her stomach for a long time to turn the baby over.  at the end assuming a girl, for me it is a girl.  I felt and was confident even a hundred boys, since I have a daughter and everything goes differently, at 18 weeks my obstetrician did not see the deal and that there was something between my legs.  who to believe?