What is the due date that you were given? If you are ten weeks pregnant, it sounds like the conception date would be around the January 7th date.
The way a pregnancy is counted, is that your first two weeks you are technically not pregnant- basically they count from the last menstrual period, and 'normally' a woman is fertile about 14 days after that- so that's when you would conceive.
It's very difficult to tell you the exact date of conception, because you don't always fertilize the egg the same day you have sex- sperm can 'live' for about 5 days- fertilization could occur anywhere in that time frame.
Also, ultrasounds are not always 100% accurate with dating- the further along you are, the less accurate they are. They can be off by weeks.
A due date is just an estimate either way- they usually guess about a 2 week margin of error on both sides. Also the HCG level has a very wide range to it, so you can't really tell how far along you are with that measurement.
Are you concerned the due date is incorrect or do you have other concerns about the pregnancy you are trying to address?