How To Confirm Conception Date During Pregnancy?

Hi dr. I had unprotected sexual intercourse on the of 12th of sept ; && the next day i started my monthly cycle. my menstrual ended somewhere around the 18th of sept but thru the 16th-28th of sept my fiancée && i had sexual relations everyday thoes days the 16th of october i went to the dr && was told i was arround four weeks pregnant && my due date is june 20th 2015 is it possible to tell if i got pregnant on 12th or on the 16-28th days ?