Ultrasounds even in early pregnancy can be a week off. Most likely it is your boyfriends since it would be 5 weeks since you last had sex that you concieved. However because the dates are so close together the only way to know is to get a paternity test when the baby is born.
They are all right. The Dr. might be able to tell you a day that he thinks you concieved based on the size of the baby near the end of your first trimester but this is going to be based on averages, so having sex with different partners on close days means there is no exact way to know except a paternity test. A Dr. can only give a best guess and that can often be off by days which would really make a difference in your situation. good luck!
When you are a few weeks along, they can tell you about how many weeks AND days you are. If you would like to use the days to make a guess that may be helpful but if you had sex with two guys on one date, there's no way to tell.
Considering you had sex with so many men in such a short amount of time the only way to know is a paternity test.
It may be your boyfriends but dont hold that to me.... The only thing you can do is wait and get a DNA test after babes is born.... in the mean time just take care of yourself and unborn. good luck.....