Explain that you need to get in for an ultrasound right away, because you have a paternity question and you know the window for an accurate assessment of the estimated conception date is relatively short. Hate to have you explain the situation but if they won't book you any other way, you've got to do it. Also make sure they understand you are willing to pay out of pocket for an ultrasound. They might just be doing standard procedure because they are only intending to do what insurance will pay for, and no insurance pays for an ultrasound in the 7th or 8th week.