When a gestational sac is visible on an ultrasound but no yolk sac is seen, it can be concerning. The presence of a gestational sac indicates that pregnancy has occurred, but the absence of a yolk sac may suggest an early-stage pregnancy or potential complications. In normal pregnancies, the yolk sac becomes visible around 5-6 weeks, providing nourishment to the developing embryo until the placenta takes over. However, if the ultrasound shows a discrepancy between the calculated and observed weeks of pregnancy, it could indicate either an incorrect calculation or possible developmental issues such as delayed growth or miscarriage. It is important to follow up with your healthcare provider for further evaluation and guidance. They may recommend additional ultrasounds or tests to monitor the progress of your pregnancy and ensure optimal care for you and your baby.