It depends  on  which  day of your  menstrual  cycle  you had sex. You are most fertile in the  mid cycle  between  day 12 to   day 17.  You can take  emergency  contraception  like I pill preferably  with  in 12 hours of  unprotected  intercourse.  If you  have  not  taken  do a  pregnancy  test  if you  miss periods.  In case it  is  positive  u can  still go for  medical  termination  up to  7 weeks of pregnancy.