Can a first trimester ultrasound be off by three weeks as far as the conception date and paternity? I have been given a due date of 09/01/2015. My now husband and I broke up last year in September. I started seeing a different guy for a couple months and the last time I slept with him was 11/10/14. I reconciled with my husband on 11/11/2014 and have been intimate with him ever since. We slept together throughout all of 11/2014 and 12/2014. I went to the hospital on 12/28/2014 and was told that I was pregnant. On 01/15/2015 I was seen in the Emergency Room and given an US that dated me at 7w1d. On 01/22/2015 my OBGYN did an US that set me at 8w2d. My DD has remained the same for 9/1/2015. I am now 17w1d pregnant. I was very surprised by this pregnancy, as my periods are so irregular that it is not uncommon to skips months of menstruation. What is the likelihood that the man from 11/10/2014 is the father of this baby?