Sperm does last in a woman's body a while, but no research I've seen says it lasts 11 days. If your ultrasounds were early in your pregnancy, it doesn't sound like you got pregnant from the sex on the 16th.
You said "conception date is usually not the day the sex occurred." While it is true that it can happen a few days later than the unprotected sex, it is also perfectly normal for it to be right on the day of the sex. Just all depends on where your egg is at the time.