For women, breast development generally occurs until about 18 years of age. The genitals and breasts undergo cyclic changes under the action of sex hormones secreted by the ovaries. However, the development of breasts in females may vary individually due to genetic, environmental and nutritional factors, and cannot be generalized. Breast development is a continuous or non-continuous development related to sex hormones. Breast development is often related to nutritional, environmental and genetic factors, and there are large individual differences. For women, breasts generally begin to develop when a woman is close to 10 years old, and develop into a mature type after about 3.5 years. Generally, since the age of 18 or so, the genitals and breasts under the action of sex hormones secreted by the ovaries undergo cyclical changes, but the breasts of women during pregnancy and breastfeeding will increase in size by hormonal influences, and do not belong to the development. If a girl is 16 years old, breasts are still not developed, it is recommended to go to the hospital in a timely manner to check, clear reasons, and then targeted treatment, after the development of breasts, should wear a bra in a timely manner, to prevent breast sagging, usually also need to keep the breasts clean, to avoid trauma, and to maintain adequate nutrition, conducive to the development of breasts.