The symptom of stomach pain on the 8th day of blastocyst transfer may be caused by physiological reasons or biochemical pregnancy. 1. Physiological reasons: the uterus may be tugged by the instruments during the blastocyst transfer, which may cause the pain sensation. In addition, if the blastocyst transfer is successful, there may also be a transient mild abdominal discomfort. 2. Biochemical pregnancy: The fertilized egg does not successfully implant in the uterine cavity. After a biochemical abortion, the embryo is expelled from the body, causing uterine contractions, which leads to abdominal pain. It is worth noting that if the pain persists or you see red, it is advisable to go to the hospital for examination and consultation to clarify the exact cause.