Embryo arrest, also known as missed abortion, refers to a condition in the early stages of pregnancy where the gestational sac does not develop normally due to various factors. Generally, the gestational sac will not begin to shrink until the embryo has been arrested for a considerable period. In the initial stages of embryo arrest, since the duration is relatively short, the trophoblastic cells within the gestational sac can still secrete estrogens, progesterone, and chorionic gonadotropin. These hormones stimulate the possible continued growth of the gestational sac in the short term. However, as the duration of the embryo arrest extends, the proliferation of the trophoblastic cells decreases, leading to a decline in hormone levels in the body. This reduces the stimulation to the gestational sac, causing it to gradually shrink, decrease in size, and possibly leading to a natural miscarriage.