We usually wait until at least 6 weeks pregnancy. This is because by then we can usually see if there is a fetus and a heartbeat. Any earlier and we may only see a fluid filled sac. This means we will need to see you again and more expense to you.
Earlier scans are sometimes done to confirm there is a gestation sac and that it is in the uterus. We just may not be able to tell if the pregnancy is viable.