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The term thelarche refers to the onset of normal pubertal phase of breast development in females.

Breast development, or thelarche, begins between the ages of 8 and 13 in girls (average 10.3 years) and marks the onset of female puberty. The breast exam should be included in the annual examination of adolescent girls not only to rule out breast pathology, but also to evaluate for the physiologic activation of the hypothalamo-pituitary-gonadal axis. Breast maturation is staged using the system developed by Marshal and Tanner in 1969, as discussed previously in Chap. 3. The overwhelming majority of breast disease in adolescents is benign and thus conservative management should be pursued whenever possible.

Puberty is essential to human development, health, and reproduction. When it occurs too early or late, there are serious emotional and physical consequences. In the beginning of the twentieth century, there was a steep decline in the age of puberty and menarche in girls which is attributed to better nutrition. More recently, however, there has been minimal change in the age of menarche, but the age of thelarche appears to continue to decrease. Given the relatively short period of time in which these later changes have taken place, it is likely that these changes are due to environmental or epigenetic factors rather than genetic alterations. Certainly, childhood obesity is of concern and a possible cause but so are medications and environmental exposures. In this chapter, we will review the current literature regarding the known impact of certain pharmacologic and environmental substances on pubertal development.