U/s on 1/20/14 gave measurement of 11w4d measuring the fetus at 4.8 cm. could 11/25/13 be conception date? what is the accurate conception date?

11/14/13: Based on the information you provided, the 'average' date of conception would be appx 11/14/13. While 11/25 could be the conception date, it's highly unlikely. Most first trimester ultrasounds are the most accurate ultrasound we have for dating a pregnancy. Typically standard deviation of error is +/- 5 days. Which would be as early as 11/9 or as late as 11/19. Thus, 11/25 is an outlier.