Can anybody help me find out when I conceived ?

Natalie

So I started my period November 20th it lasted 5 day's I didn't find out I was pregnant till Christmas Day I made an appointment with the dr and she told me they go by the first day of my last period but when I did the ultra sound it was way off from the first day of my last period so they gave me a new due date which is September 5, 2018 but the first due date was august 27, 2018 so now I'm just wondering when I conceived?