It is not uncommon for fetal activity to not be visible at 5 weeks and 3 days of pregnancy, especially when only using an ultrasound. At this stage, the gestational sac and yolk sac are typically visible, while the fetal pole may still be developing. The absence of fetal activity does not necessarily indicate a problem as it can be too early to detect it. It is important to follow up with your doctor for further monitoring to ensure proper development in subsequent ultrasounds.