Had an ultrasound on dec 28th,2013.. doc says i was 6wks6days.. when was it that i had to have sex to get pregnant?

Conception: Conception calculators and all kinds of tools to understand and explain your pregnancy are readily available on the internet. Your baby was conceived on 11/25/2013 from sex that occurred sometime between 11/20/2013 and 11/25/2013. You can verify this yourself here: http://www.babymed.com/conception-calculator.

Nov. 23-26: First trimester ultrasounds are extremely accurate: if yours was clear and reliable you must've conceived in the window above, theoretically you could've gotten pregnant from intercourse even 1-2 days earlier.