Ultrasound can detect a gestational sac as early as 4 weeks with a yolk sac to follow about 5 weeks and a fetal pole at about 6weeks. The problem is you can not got on you LMP alone. Woman have different cycle lengths and even a small difference in dates can make a difference on what is seen on an ultrasound. The most accurate way to date a very early pregnancy is to have your Dr draw blood and run a Quantitative HCG. This will meassure the amount of pregnancy hormone in your blood and give him or her a very good idea as to when something should be seen for sure on an ultrasound. It can also be repeated over a few days and the Dr can tell if the pregnancy is progressing normally by how often the number double. Hope this answer is helpful.
at 6:43 PM on Mar. 18, 2011