ultrasound question

I had my first ultrasound on dec 21 and was told I look 17 weeks pregnant. In September I was sexually assaulted and have no recollection of the night, if something had happened that night to me I would have been 12 weeks pregnant. & I wanted to know how accurate it could be at 17 weeks considering I have no idea of when my last menstraul cycle was. & every ultrasound after has been right on track until my 29 week ultrasound where the technician told me I was measuring 30 weeks. 

Any feedback is GREATLY appreciated