Am I having 100% havinf a boy??

So I got my first and only ultrasound at 17 weeks. They said that I was having a boy. My family believes I'm not having a boy that they mistook the penis as the umbilical cord and wants me to ask and figure out a way for another sonogram since I am now 21 weeks. Has any of you women had or is having a boy that can confirm that this is indeed a penis?