Since the early stages of pregnancy represent an exceptionally wide range of time, their ultrasound manifestations can vary greatly from period to period. If the so-called early stage of pregnancy is within 35 days of pregnancy, due to the particularly short period of menopause, ultrasound will often only find a thickening of the lining of the uterine cavity, but no other manifestations. This is because the fertilized egg has only just laid in the uterine cavity and has not yet grown significantly into an embryo, which is why the ultrasound is unable to see a visible gestational sac in the uterine cavity. For women who are pregnant within 35-40 days, the ultrasound may find a cystic dark area in the uterine cavity at this time, just because the yolk sac in the embryo, as well as the fetal heart and buds have not yet appeared, so it can only be seen under the ultrasound as a cystic dark area. If a woman is pregnant for more than 45 days, an ultrasound will reveal a visible gestational sac in the uterine cavity.