The breast remains quiescent in the prepubertal stage and undergoes major changes at puberty under the influence of ovarian hormones. Estrogen increases adipose tissue and initiates stromal and ductal growth. Progesterone is probably important for lobular-alveolar development. These hormones produce the changes seen in the pubertal breast as documented by Marshall and Tanner.

The adult breast has a major duct system leading to the terminal ductal lobular unit, which has been described in detail by Wellings et al..

Comprising extra- and intralobular terminal ducts and the lobules arising from the intralobular terminal ductule, it is an important localization in the origin of many breast diseases.

The appearance of the breast bud or mound is the first prepubertal sign of the beginning of the estrogen ovarian activity. This development is often asymmetrical. The mean age of appearance is 10.9 (8.5 - 13.3) years, and other secondary sexual characteristics follow, including hair growth and morphological modifications of the vulva. The duration of mammary gland growth is approximately 2 - 3 years, although sometimes longer. The first menstruation occurs usually 2 years after the S2 period.

Table 2 shows the different Tanner stages leading to adult stage with a surrounding breast and a protruding nipple.

In the newborn, presence of a breast bud is frequent and characterizes neonatal mastitis, a sign of passive in utero hormonal impregnation. In late pregnancy, high levels of luteal and placental hormones in the mother’s blood cross the placenta into the fetal circulation and stimulate the fetal breast. This breast bud disappears spontaneously after some weeks. Hormonal manipulation delays this regression and involves a risk of infection (breast abscess); similarly, surgical treatment can definitively damage the future mammary gland.

During childhood before 8 years, the development of a breast bud is a warning sign. Indeed, premature thelarche occurs unexpectedly in 70 - 80% of cases before 2 years. The breast bud can be uni- or bilateral, but the areola is not modified. Breast development is isolated and not accompanied by other pubertal signs (absence of hair growth, prepubertal smear test, absence of growth acceleration and normal bone age). Spontaneous regression is common but clinical follow-up is necessary during the first months in order to detect premature puberty.

During puberty, breast elevation increases at stage S2 and growth and protrusion of the nipple appear. The breast is often painful either spontaneously or on palpation. This palpation is sometimes not easy, especially in stout children. Unilateral breast development should not necessarily suggest pathological tumefaction and does not justify aggressive investigation (biopsy for example), which carries the risk of damaging the future mammary gland.

Throughout the growth period, breast palpation often remains difficult.
The mammary gland is firm and tender, with a multinodular aspect making it difficult to distinguish from pathological tumefactions. It is often useful to repeat the examination, preferably in the post-menstrual period. Clinical examination is essential because generally it will lead to diagnosis without complementary investigation. Ultrasonography is the usual complementary investigation in adolescence. Mammography remains exceptional because the high mammary density at this age results in a uniform opaque zone that cannot be interpreted.