What is the lowest number of days it is possible after conception, to see a gestational sac with cells attached to the wall,on a sonogram/ultrasound?

Once betaHCG >1000: Blood levels of betaHCG are better and often more reliable than last menstrual period (LMP) or suspected "conception day" and best correlated with the sonographic findings of current transvaginal ultrasound. Once the betaHCG levels pass 1000mIU/ml, most skilled sonographer may detect a small intrauterine gestational sac , as a first sonographic finding.