I understand your concern about paternity. I am sorry but calculating the day of conception by using your last menstrual period will not be exact due to the proximity of both sexual encounters.
In my opinion the only way to know who the father of your baby is; a prenatal or postnatal Paternity test would be the only exact method to give us the answer.
This could be done as early as 10 weeks of pregnancy by Chorionic Villous Sampling or later on with an Amniocentesis. If you would like to wait after your child is born, a buccal swab or blood sample may be a less invasive way to obtain the result.
These Q&A’s are for educational purposes and should not be relied upon as a substitute for medical advice you may receive from your physician. If you have a medical emergency, please call 911. These answers do not constitute or initiate a patient/doctor relationship.