It is interesting that you have read that ultrasounds only have a margin for error of three days as late as 15 weeks. I've heard from numerous people on this site who had been told by their doctors that at 12 weeks there is a margin for error of +/- 7 days or more. Several times they have quoted this from their ultrasound reports.
Anyway, unless you are going to the appointments with her and sitting in while the doctor does the ultrasound analysis, there really is no way to know. It doesn't sound like your boyfriend's baby, unless something about the dates he had sex with her are not what he told you. But it does sound close enough that a DNA test is necessary to know for sure.
But I think she used the date of her last period at the first appt and then they did the ultrasound at her 2nd appt and compared....
I believe at that point she was 13 weeks and a few days...I've read that up until about 15 weeks, ultrasounds are pretty precise give or take 3 days. With his situation there is a 9-10 day difference....
If the ultrasound was from her sixth or seventh week (counted from the first day of her last period), he could stop worrying so much that the baby is his, because the estimated conception date from her due date does not line up with when he [says he] had sex with her. But if her first scan was not until a few weeks later than December 3, that sounds too late for certainty in determining an exact conception date. Do you know how many weeks and days she was at the time she had her first scan?
She had originally confirmed pregnancy dec 3rd at the dr and had her first scan a few weeks later
The dates make it sound like the baby was conceived around October 4 or 5. How early did she get the dates? An ultrasound in the 15th week can have a pretty big margin for error if someone is trying to use it to determine paternity.