It is possible for a pregnancy to be miscalculated if there is no sac visible at 6 weeks, despite calculations based on the last period. This could occur due to factors such as irregular menstrual cycles or delayed ovulation. In some cases, it may simply mean that the pregnancy is not progressing as expected. However, it is important not to jump to conclusions and consult with your healthcare provider for further evaluation. They may recommend additional tests such as an ultrasound or blood work to determine the status of your pregnancy accurately. Remember that every woman's body and pregnancy journey are unique, so try not to worry excessively until you have more information from your healthcare provider.