The yolk sac can be seen as late as around 10 weeks. The yolk sac is the first structure to differentiate from the blastocyst cell mass and is the earliest content of the gestational sac that can be visualized on ultrasound. The yolk sac usually appears at 5 weeks of gestation and continues to grow as the embryo develops, becoming smaller and eventually disappearing between 8 and 10 weeks of gestation. The yolk sac is normally seen at 5 to 10 weeks of pregnancy. Irregular periods, hormone deficiencies, and viral infections can cause the yolk sac to form later. Usually women will grow yolk sacs when they are pregnant, but a few women will have empty sacs when they are pregnant and will not grow yolk sacs or fetal heartbeat at a later stage, which requires medical attention to check whether they are pregnant. It is recommended that women do regular pregnancy tests after pregnancy to keep abreast of the development of the fetus, and should consult a doctor in time for treatment if any discomfort occurs.