How To Determine Paternity Of Child?

Iam 21 iam not pregnant yet but I'm scared of not knowing who the father might be .. I have a regular monthly period I recently had sex with my ex boyfriend on January 7.. My ovulation days are january 11-21 .. I had sex with boyfriend on those days of my ovulation days .. Iam scared of not knowin who the father is if I was to become prego