Around what date did i conceive?

my doctor put me on provera to start my period in December so i don't know if i have 28 day cycles but i had my period December 13th or 14th and i went to the doctor today and she said based on my sonogram measurements im 12 weeks and 2 days. she said the sonogram was very accurate. so around what date could i have conceived? i had with my Ex on December 20th & 26th and my guy friend on the 29th but on the 29th i took the morning after pill right after . Thanks!