Hi. I had sex with 2 different partners about 3 weeks apart. Intercourse with the first was ended on August 4th. I started and continued having with the second about 2 to three weeks later on a frequent basis afterwards. I found out I was pregnant on October 15th and had an ultrasound report which stated that the fetus was less than six weeks. No visible yolk sac and no fetal pole. By my calculation that should mean that conception took place around September third. What is the possible margin of error and how certain can I be that guy#2 is the father. I cannot recall my last menstrual period. Though I did experience spotting around mid August.

Dating is all Best Guess I'm afraid.
If there is any doubt over paternity then a DNA test would be the only sure route to take.

The dating is normally give or take about two weeks.

So while this is slightly out of range, I would recommend a DNA test to be sure after the baby is born

As Ben has said, you will need to have a paternity test once the child is born.