My sisters pregnancy

This is my sister's 3rd child, who will be born November 29th 😊. Well she had her 1st dr appointment August 30th at 6 months. In the ultrasound it showed the placenta being too low, they said it was common and that it usually goes up. She asked me if its ok, if that was normal but I couldn't tell her nothing since didn't experience that in both my pregnancies. Does anyone here know if this is ok? What happens if it doesn't go up? Also she tested for positive for diabetes, she hasn't told me anything, my mom was the one who told me so I do not have much info on that. But I'd love to know more about the placenta