The 12mm*12mm gestational sac without yolk sac may be the pregnancy time is too short, has not yet developed the yolk sac, or may be the embryo is arrested, so there is only the gestational sac and not see the yolk sac. The 12mm*12mm gestational sac without yolk sac, this situation may be the pregnancy time is still too short, ultrasound can only see the gestational sac, yolk sac development is not yet to the extent that can be seen. It is also possible that the embryo has stopped developing and the ultrasound can only see an empty gestational sac, not the yolk sac. In this case, the pregnant woman can wait for another week for a repeat ultrasound and blood HCG, and if the yolk sac can be seen on the repeat ultrasound, the pregnancy can continue. If the yolk sac is still not visible in the ultrasound and the HCG is abnormal, you can consider that the embryo has stopped, and you need to clear the uterus in time. If there is any abnormality during pregnancy, you need to consult a doctor and follow the doctor’s advice.