The absence of a yolk sac may be caused by physiological reasons, poor embryonic development and other reasons. 1. Physiological reasons: with the increase of gestational weeks after pregnancy, ultrasonography can clearly see the intrauterine gestational sac and clear intrauterine pregnancy. However, when the pregnant woman has a long menstrual cycle and late ovulation, no yolk sac can be seen on ultrasound due to the short duration of pregnancy, while progesterone and blood HCG (human chorionic gonadotropin) tests are normal, and the test can be repeated in 1 week according to the doctor’s instructions. 2. Embryonic dysplasia: pregnancy has been confirmed during early pregnancy by urine or blood HCG test, and ultrasonography also shows the gestational sac in the uterine cavity, but when embryonic dysplasia occurs, there is no yolk sac, and there will not be fetal buds or fetal heart, which is a kind of empty gestational sac, and it is necessary to terminate the pregnancy in time after the doctor’s full assessment. When the yolk sac is not seen on ultrasound, it should be observed and rechecked under the supervision of the doctor.