Understanding Pregnancy Timeline: Conception Date & Due Date Discrepancies

Question
I need your help. I had a one night stand with this girl on 1/17/12 @ 3am. 8 days later on 1/25/12 she told me she was pregnant.  I couldn't  believe it.  She had a ultrasound done in the beginning of February not sure of the date but the baby measured 6w6days with a due date of 9/30/12 she had a 8 week, 12 week and 18 week ultrasound with the due date remaining the same . But i am a little confused and so is my parents. The due date giving is placing conception around january 8th. When i asked her she said it is possible that the due date and ultrasounds can be off by 10 days and that implantation happened right way thats why she tested positive so quickly.  I saw the ultrasound pictures and the due date is 9/30/12 and LMP was 12/25/11. and i recall her saying her cycles are irregular but never longer then 32 days. so the ultrasounds can be incorrect I am having doubts that the baby is mine. And my mother agrees that the baby is not mine. Please help. I am very confused! I will feel bad if i ask for a DNA test and the baby is mine."

Answer
Mike

It is really hard for a single intercourse on January 17th to be visualized at the beginning of February measuring almost 7 weeks. It is true that a difference up to 2 weeks may occur in estimating gestational age; a difference between ultrasound dating and dating by last menstrual period. However; this is for a 37-40 weeks pregnancy. At 7 weeks, such discrepancy is much less. At the same time, the DNA paternity testing remains the only sure method to diagnose paternity if in doubt.

I hope these info would help you get an answer for your worries.