The diarrhea of a three-month-old exclusively breastfed baby is mainly considered to be breast milk diarrhea. The reasons for breast milk diarrhea may be the following: 1, the prostaglandin content in breast milk is high: the substance promotes the smooth muscle movement of the small intestine, increases the secretion of water and electrolytes and produces thin stools, the infant organism will gradually adapt to the prostaglandin in breast milk. 2, the baby is intolerant to lactose: the body lacks lactase. Such babies can be consumed several times in small amounts to reduce lactose intolerance, and the amount of each serving should not exceed 250ml, or they can be fed with low lactose milk or rice soup, which can greatly relieve their lactose intolerance. 3, the fat content in breast milk is high: pay attention to the mother’s diet, if the mother often eats more oil, high fat, stimulating food, resulting in the milk contains more than the baby’s needs, it will make the baby diarrhea. In addition, the mother’s diet needs to pay attention to the baby’s food allergies. For example, if the baby is allergic to milk, eggs, seafood, wheat, etc., then the mother will also experience increased diarrhea and even blood in the stool if she eats these foods, and some babies will have significant eczema. If the mother’s diet adjustment is still not enough to improve the baby’s diarrhea, she can give her baby some lactase to help digest the lactose in breast milk. In the case of physiological breast milk diarrhea, do not switch to cow’s milk in order to form the stools. In mild cases of physiological diarrhea in newborns, no treatment is needed and nursing can be continued so that the lactase enzyme will gradually develop and mature, and the enzyme activity will increase so that it can gradually break down, digest and absorb lactose and gradually improve with the addition of complementary foods. If severe diarrhea causes growth retardation, malnutrition, etc., prompt medical treatment is required.