Paternity Test & Due Date Calculation: Understanding Conception Dates

Question
Hello,

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
From the information you gave, she got pregnant around January 7. First trimester ultrasounds (before 12 weeks) can be off by a day or two, but they are not off by 10 days. There is too much development going on during that time for them to be off by that much. Also it is highly unlikely that she would have a positive pregnancy test 7 days after intercourse.

Definitely get a DNA test. I think it is almost impossible that the baby is yours. If the baby is yours (extremely unlikely) then it is still good that you got the DNA test to make 100% sure.