Wrong Due Dats

Quick Question Everyone. My friend is pregnant and has like 3 different due dates. Her dr is telling her she is 34 weeks by how her belly is measuring, period says 36 weeks and ultra sound 37 weeks. When I was pregnant I was always told to go be your period. But she is saying she was told to go by the ultrasound, which I don't think is correct.  What is your opinions?