Hello and welcome, I am Dr Nilajkumar a consultant gynecologist and I will be helping you in your queries. If the last menstrual period was feb 12, the day of ovulation would be around feb 26. The fertile period ranges from feb 21 to to march 3. So any act of sexual contact during this time may have lead to pregnancy. It would help to know on what date you are 3 weeks 4 days otherwise unfortunately it is not possible for me to calculate. Nevertheless in any case, even with ultrasonography or blood tests for diagnosing pregnancy, it is not possible to accurately predict about paternity and date of conception as the machine itself has an error of 1 week on both sides. I hope this will clarify the issue of the pregnancy being 3 wk 4 days. So we can never be sure with these. The only way to do this is by a paternity testing using genetic analysis. This can be done during pregnancy or after delivery where the chromosomes of potential fathers are matched with those of the baby. I hope that you will realize that no amount of calculations will be accurate. The only way is genetic tests. I hope I answered it to the best of my capacity and have managed to satisfy your query. Thanks for using HCM. Have a good day. Dr Nilajkumar Bagde Consultant Obstetrics and Gynecology