Dated 12 weeks 4 days on 7th aug (first trimester), no lmp, how accurate is crl? and is this age from conception?

Fairly accurate: The accuracy of a CRL in determining your gestational age and in estimating your due date at this point in your pregnancy is fairly good (+ or - 5 to 8 days) if the sonographic exam was performed properly by a certified sonographer. Although the margin of error of ultrasound increases as the pregnancy advances, it may be valuable to you to repeat your exam in 4 to 6 weeks to confirm your dates.

Very accurate: Determining gestational age by crown rump length during the first trimester of pregnancy is very accurate. The determined gestational age pretends that there was a menstrual period that started in this case 12 weeks plus 4 days before the day of the ultrasound. Conception would have been about 2 weeks after this assumed first day of LMP.