9 months old baby stool frequency can be seen in gastrointestinal dysfunction, lactose intolerance, acute enteritis and so on. 1. Gastrointestinal dysfunction: such as for 9-month-old babies have been gradually added to the complementary foods, such as the addition of improper complementary foods, there can be an increase in the number of stools phenomenon, part of the vomiting can occur, usually less severe symptoms. 2. Lactose intolerance: the relative or absolute insufficiency of lactase enzyme in the intestinal tract caused by various reasons makes the lactose-containing food cannot be digested and absorbed, and then the above phenomenon occurs, and some of the babies may have poor weight gain in a more serious way. 3. Acute enteritis: mostly caused by viral or bacterial infections, the number of stools can be significantly increased, may be accompanied by changes in the character, such as presenting dilute water stools, may be accompanied by vomiting, fever and other symptoms, and in severe cases, may appear less urine and other dehydration phenomena. 4. Other: such as acute upper respiratory tract infection, may appear body temperature increased significantly, affecting the gastrointestinal function of the phenomenon described above. It is recommended to consult a specialist to assess and determine the cause.