Ultrasound on 4/12/13, i was 9w4d doc say i conceived on 2/1/13. but 2/1/13 was the 4th day of lmp(21days cycle) could ultrasound be off a few days?

A 9wk 4 day: Us can be off 2-3 days either way.

Yes: Ultrasound is most reliable at 6-7 weeks when it predicts gestational age to within 5 or 6 days. As the pregnancy progresses, accuracy of ultrasound gets poorer. At 18-22 weeks, ultrasound is accurate to about 10-12 days. In the third trimester, ultrasound dating can be off by as much as 3 weeks.