An ultrasound done within the first trimester is pretty accurate and can be off by a week. If you take away a week, that would put the conception date at Dec. 30; if you add a week that would put you at Jan. 13, before you had sex with the other guy. I would get a test to be on the safe side and for my own peace of mind though. There's a new technique where they take a blood sample from the mother and a blood sample, hair or cheek swab from the potential father. There is no risk to the baby but I don't know that results from this new procedure are admissable in court (child support.) Maybe you should research and consider doing this. I believe you can take this test as early as at 13 weeks.

Thanks, but does anyone anymore info on this? I mean I've heard ultrasounds can be off by two weeks is that true? My doctor told me that it could be off 7days earlier and 7days after date of conception from my ultrasound. And if I had sex while on my period can you get pregnant other then your ovulation date?

To my knowledge, an ultrasound is accurate within 3 to 5 days. I am in a similar situation and been doing research into this. Every doctor I have spoken to thus far says that the ultrasound is very accurate and the time frame of 3-5 days is just a safety measure.

Goos luck