It is possible for a scan at 6 weeks to not show a yolk sac, but it can be concerning. The presence of a yolk sac is an important milestone in early pregnancy as it indicates the development of the embryo. However, it is also important to consider factors such as the accuracy of the gestational age and the quality of the ultrasound machine used. In some cases, the yolk sac may simply be too small or difficult to detect at this stage. Your doctor may recommend follow-up scans to monitor the progress and determine if there are any other signs of pregnancy viability. It is best to consult with your healthcare provider for further evaluation and guidance based on your specific situation.