If your due date is 28th august 2015 by ultrasound , then by Naegle's formula your date of conception should be 21 november 2014 or a week beyond that ie 27 nov 2014. There is always a discrepancy of one week in actual date of conception and ultrasonological inference. So the conception should be between 21-28 th november 2014.
I hope i have answered your query,
wishing you safe pregnancy,
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