Hello and welcome to the forum. I am Dr Nilajkumar a consultant gynecologist and I will be helping you in your queries. The chances of you getting pregnant while on minipills is very low. However they do not hinder ovulation. So the first sexual episode is not likely to have caused this pregnancy. Also you had used the pills for two more days after sex making it difficult for the endometrium to recover from the effect of pills so early as to permit implantation even if pregnancy occurred. It would have helped to know the date when you are 23 weeks to help get a better estimate of date of conception. Nevertheless in any case, even with ultrasonography or blood tests for diagnosing pregnancy, it is not possible to accurately predict paternity and date of conception as the machine has an error of 1 week on both sides. The only way to do this is by paternity testing using genetic analysis. This can be done during pregnancy by amniocentesis 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 was able to help you. Thanks for using HCM. Have a good day. Dr Nilajkumar Bagde Consultant Obstetrics and Gynecology