Baby measures smaller

So I'm 11 weeks pregnant. But my ultrasound said the baby measures 9 weeks and 1 day. Is this when I conceived? Or how can I find out when I conceived by this new date? I have a 39 day average cycle. And when I go back exactly 9 weeks and 1 day it goes to around the 20th of August but I didn't have sex that day or prior?

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