If you have a tilted uterus (one which angles towards the back instead of the front) then very often they can't pick up the baby with an external US in the first trimester and have to use an internal one to get a good scan.
This generally has no effect on the pregnancy, it's just a bit annoying and invasive - I had to have internals done with all three pregnancies in the first tri.

With regards to when you see your obstetrician for the first appointment, a lot has to do with where you want to have your child. Some of the private hospitals in the larger cities have limited places, so you need to be booking in at six weeks to be certain of a place, and only your ob can book you in which explains why some women rush to get that first appointment in.

Kerrie