It is possible for the baby not to be visible during a 6-week abdominal scan, especially if you did not have a full bladder. A full bladder helps push the uterus up and makes it easier to visualize the baby. If your bladder was not full, it may have affected the visibility of the fetus. However, it is important to consult with your healthcare provider for further evaluation and reassurance. They may recommend additional tests or scans to confirm the viability of your pregnancy.