I am an ultrasound tech and to be completely honest, we like to do an internal exam for cervical length becuase it is the best way to visualize it, especially the further along you are. The babies bones can create shadows that can hide your cervix, but by doing an internal exam, it puts our camera right up next to your cervix.
Hope this helped.
Yes, it gives almost accurate result that's why it's most preferred way to measure cervix length.
I had my cervix measured during 20 week scan. She just had to put the transducer down low and push fairly hard to see the cervix.