If you go to that website
www.debbiejensen.com you can put in the breeding date and it will give you a whelping date (due date).
I'm estimating that if her heat started 6 weeks ago, she probably bred about 10 - 14 days after that, so she would be about 4 weeks along. You might want to save your money on the ultrasound because very soon you will be able to see the weight gain anyway. I really like giving an X-ray at the very last about Day 50 because you can usually tell how many to expect and also see if there are any larger pups that might require a C-section.