Dr Megha Sarakshi Chadha

Mam if we don't know last period date then how can I find when did I actually get pregnant

Because I think my last period on march 14 after that I had intercourse on apirl 5 only one day then my next period date on apirl 15 but due to exam stress and other issues I don't know can I get period or not. After that may and june I surely missed period but may 7 onwards I had Intercourse continue many days but may 28 ultrasounds show 7 week and 2 day only and due on February 13.then when did I actually get pregnant.