The farther along you get, the more accurate the ultrasounds get, but usually, they are pretty accurate from the beginning. It just depends on how far along you are when the ultrasound is done.
It gives the ultrasound technician a peek at the fetus, to identify certain key traits that can gauge how far along the pregnancy is. For instance, the presence of a 'yolk sac' is indicative of a...
With my first child I was 7w3d when I had my first u/s. My second child I was 9 weeks. This coming Monday I will have an u/s @ 6w6d and hope to see a heartbeat. I heard from my Ob/Gyn that at 6w1d...