Don't go by the ultrasound dates. Like everything, they are estimates. Fetuses, just like babies and children, develop at different rates.
Most doctors base dates on when your LMP occurred. When did your last period start?
Having sex, within about 7 days with two partners, you ARE going to need a DNA test to determine paternity. The problem is that sperm can survive about 5 days. Conception does not occur immediately either.