Hi, Thanks for the query. I understand your problem. If we calculate from your last menstrual period, assuming that you had regular periods, the time of ovulation is most likely between 6th to 12th. Also, the sexual episode on 16th was incomplete. So, the chances of pregnancy if proper penetration has not occurred are low. The plan B, may rarely delay ovulation but it prevents pregnancy by making the cervical mucus very tough for sperms to go through and even if the sperm does manage to go through, the baby cannot stay in the uterus as the conditions inside are made unfavorable by plan B. This again is a safeguard against pregnancy. So the pregnancy was most likely already in place when plan B was taken. All of this favor that the pregnancy resulted from sex with your fiancee. The ultrasound result depends on many things and if it was done through the abdomen or vaginally. The report provided by the ultrasound machine has an error of + / - 1 week when done in the first three months of pregnancy. So a small difference of 3-6 days is always there by ultrasound. It is difficult to guarantee that your fiancee is the actual father, but considering the scenario he is the most likely father. The best way to determine this would be genetic testing, a chromosomal analysis of amniotic fluid is possible but will need to be compared with the chromosomes of both the potential fathers. But still, from your complete history, I feel that your fiancee is most likely the actual father. I am sorry that it is not possible for me to provide a more definite answer for this as it needs further testing. Thanks for the query and do not worry. Dr. Madhuri Bagde Consultant Obstetrician and Gynecologist.