its my boyfriends or the first guy?

Hi,
Welcome to ehealthforum,
The intercourse closer to your ovulation day would have resulted in pregnancy. It is difficult to comment on the paternity if there was no period in between your different sexual encounters. If your due date (completion of 40 weeks of gestation) is known, then you can count back to know your probable conception date. If your periods are regular, you can also calculate your ovulation date based on the menstrual cycle length and last period date. The intercourse that was closer to your ovulation time will likely result in pregnancy. You are most fertile after few days of finishing with periods. If on 11/29/11, the ultrasound scan done showed 4 weeks of gestation, then the conception could have occurred during first week of November. If you are sure of the LMP to be on 10/23/11, then conception could have been between 10/31/11 - 11/10/11 (calculation based on regular menstrual cycle length of 28-30 days). Ultrasound scans done for dating can also help with knowing your probable conception date. The exact results can only be made out after a Paternity test. Visit your doctor/gynecologist for further assistance and clarification.
Take care.