### instruction:
If you are a doctor, please answer the medical questions based on the patient's description.

### input:
Hello, I started my period on June 4th and ended on the 8th and I had sex with one guy that night. On the 11th of June I had sex with another guy but he put on a condom 5 minutes after we started to keep from nutting inside of me. I had sex with the first guy again on June 15th. I later had sex with the same guy that whole next week. I missed my period for July & when I took a pregnancy test on July 6th I was pregnant. I went to the doctor and they said I was approximately 8 weeks pregnant. My due date was March 7th 2015 but when I went to get an ultrasound on August 11th my doctor told me my due date was now March 11th 2015 and I conceived between June 13-15th. So who is the father? I believe I got pregnant on July 18th because of my first due date on March 11th. I just think my baby is growing fast so therefore the CRL measurements of my baby made the doctor change my due date to March 7th.

### output:

Hello, and I hope I can help you today. Unfortunately, ultrasound findings of the dating the pregnancy can be off for up to five days. So the changing of your due date by four days does not really change the possibility of your first partner having been the father of your baby. Furthermore, sperm can remain alive in your genital tract for up to five days, and once you ovulate, the egg remains alive inside you for up to 72 hours. This is the reason that no test can accurately assess the date of conception based on the exact dates that you had intercourse.  Women typically ovulate two weeks before their period arrives.  If you have a 28-day cycle, and your last period started on June 4, you likely ovulated around June 18. So either of the men you had sex with between the 15th and approximately the 20th could be the father of your baby. I hope that this information was helpful to you and that I was able to adequately answer your question even though it is not possible with medical science to determine the paternity of your pregnancy. I wish you the best of luck with your pregnancy and good health. Best wishes,