Gender Guess

How possible is it for sex to change at 20weeks. I had an ultrasound at 15w, and the woman said hesitantly that it’s a boy, after saying she can’t really say at that time. She showed me something like a Willy, I didn’t really pay attention as I was kind of not so happy it’s a boy, I was almost 90% positive it’s a girl ( I have one boy already) my anatomy scan is in few days and I am so full I hope it was the umbilical cord she mistook for a penis. I know it’s silly but I would be over the moon if it’s a girl.