Is it possible my ex is the father? last period was june 27th-1st of july. we had sex on the 3rd of july.

Conception: From the first day of menses to the next first day of menses is the cycle length. Cycle length minus 14 days is most likely time of ovulation. Ex: 28 day cycle minus 14 days equals day 14 as most likely time of ovulation. Since you had sex on cycle day 7 (day one is first bleeding day), odds are low that you were ovulating on the date of concern.