When did I concive I'm due march 20 and my last perido was may 31 2011

My last perido was may 31 I had sex June 11 12 14 and on and I just want to know when did I concive and around what dates I had to have sex to concive

The date of conception is NOT an exact science but a best guess.
If you have issue of paternity then you will have to wait until the birth and take some DNA tests.