A woman’s flat chest may be due to heredity, malnutrition, breast compression, endocrine disorders and other reasons. 1. Heredity: flat chest may be related to heredity. In addition, black Africans, white Europeans and Americans generally have larger breasts than the Asian population, which may be related to ethnicity. 2. Nutritional deficiencies: Breasts are mainly composed of mammary glands and adipose tissue. When nutritional intake is insufficient, especially the lack of carbohydrates, fats and other nutrients, it will lead to too little fat and manifest as flat chest. 3. Breast compression: wearing tight underwear for a long time, sleeping prone for a long time, etc. leads to breast compression, poor local blood circulation, and the distribution of fat tissue is not concentrated, so flat chest. 4. Endocrine disorders: estrogen has the function of promoting the proliferation of breast ducts, and progesterone has the function of promoting the development of breast follicles. Endocrine disorders lead to insufficient secretion of estrogen and progesterone, breast development is insufficient, and thus flat chest. A woman’s flat chest may be related to a variety of factors, it is recommended to consult a doctor for a clear diagnosis.