In a normal woman, the uterine cavity is a hollow organ without any tissue. If a small sac is found in the uterine cavity under the ultrasound, it is highly suspected that the patient is pregnant. This is because in the early stage of embryonic development, when the fertilized egg is laid in the uterine cavity, the cells will keep dividing and then the embryo will grow slowly, and at first you can see the small sac-like tissue under the ultrasound. The germ is the heartbeat of the fetus found inside the uterine cavity.