It is common for an ultrasound at 6 weeks and 2 days to show only a yolk sac without a visible baby. This can occur if you are still in the early stages of pregnancy or if you ovulated later than expected. The yolk sac provides nourishment to the developing embryo until the placenta takes over. In most cases, a follow-up ultrasound after a few weeks will reveal a visible baby with a heartbeat. However, if there are concerns about your pregnancy, it is always best to consult with your healthcare provider for further evaluation and guidance.