The pregnancy sac is not visible in the uterus under ultrasound at 6 weeks of pregnancy may be caused by late conception, ectopic pregnancy or early miscarriage. 1. late conception: irregular menstrual cycles, 6 weeks after menopause to detect pregnancy, ultrasound can not see the gestational sac, may be the late conception, the gestational sac has not yet bed or just bed, ultrasound can not be seen for the time being. 2. Ectopic pregnancy: If the gestational sac cannot be observed in the uterus, it may be due to the presence of the gestational sac in the fallopian tube or other uterine attachments, resulting in ectopic pregnancy. 3. Early miscarriage: If the gestational sac grows and develops poorly in the early stages of pregnancy, it may result in early miscarriage, and there may be a small amount of vaginal bleeding, and the gestational sac cannot be detected in the ultrasound result. If the patient is not feeling well, it is recommended to consult a doctor for a clear diagnosis and targeted treatment or therapy with the help of a doctor.