I went to the doctor on September 24 and had a transvaginal ultrasound done. They determined I was 9w4d. So about a 10-11w gestational period. My problem is that I had sex with my normal partner on July 6th and on July 7th I was taken advantage of by someone else. I'm pretty sure I had my period around the 16th but I'm worried it was the egg implantation. I then had sex again with my normal partner on July 26th the doctors determined this is the time conception occurred and my due date was April 22nd 2014 I told him the baby was his but I'm so worried it's the other guys. How far off can the doctors be?

The ultrasound is only accurate to a plus or minus two week period... this is because of the accepted range of variance in fetal development, as well as the fact sperm can be in the body for up to a week before the pregnancy occurs.. Any partner inside that 4 week period can possibly be the father.

The Ultrasound is normally good, give or take a week, but can be off as much as two weeks.

An ultrasound is good for two weeks either way. You are going to have to have a DNA test after this baby is born to determine who the father is.