It is quite common for a pregnancy not to be visible on a scan at such an early stage, especially if it is an abdominal ultrasound. At 3 weeks and 5 days, the embryo is still very small and may not yet be detectable. In some cases, transvaginal ultrasound may provide better visibility. It's important to remember that every pregnancy is different, and the timing of when the pregnancy becomes visible can vary. If you are concerned or have any doubts, it's best to consult with your healthcare provider who can provide more accurate information based on your specific situation.