It takes about ten days for the dark area in the uterus to transform into a gestational sac, and there is some individual variation in the timing. When you have an ultrasound early in your pregnancy, around five weeks, you will not see a visible gestational sac, but only a dark area in the uterine cavity. In some cases, it may take a week before the ultrasound is done again and the sac is visible, or it may take more than ten days before the sac is visible. A visible gestational sac with a yolk sac and embryonic buds is certain that the embryo is developing normally. If the yolk sac and embryo buds are not visible on ultrasound after eight weeks or more, and the gestational sac is irregular in shape and small in size, it is likely that the embryo has stopped developing. Further examination should be done to determine that the embryo has stopped developing, and then it is necessary to take timely measures to clear the uterus. If there are any uncomfortable symptoms during pregnancy, you need to consult a doctor in time.