baby low?

So my friends a heavy set lady. Had a c-section and is pregnant. Well she's 13 weeks. They have to do a vaginal ultrasound everytime she goes in. could it be because she's heavier set and has that fat layer with the c-section or could it be because the baby is sitting low? i'm not being judgmental but she's nervous about it. She's asked her doctor but the doctor didn't say anything about it.