depends on timing of unprotected sex and ovulation. Ovulation occurs approx 14 days before due date of next period.
Generally D11 to D19 of regular 28-30 days cycle are unsafe/ fertile period. So you can calculate your ovulation time & fertile days. Also you can estimate with whom you did unprotected sex at that time period. Provide more information for justification as some data is missing.
For paternity confirmation, best and ideal method is DNA testing and nothing is superior to that. And DNA does not transfer/ change in between two persons. To every person, DNA typing is unique.
and enjoy pregnancy. Be well.