Thelarche is the first physical change of puberty in about 60% of girls, and is a result of rising levels of estradiol. It is typical for a woman's breasts to be unequal in size, particularly while the breasts are developing. Statistically it is slightly more common for the left breast to be the larger. In rare cases, the breasts may be significantly different in size, or one breast may fail to develop entirely.
Although breast development can occur as a part of normal male puberty, it is termed gynecomastia, and the term "thelarche" is not used with reference to male breast development.
When thelarche occurs at an unusually early age, it may be the first manifestation of precocious puberty. If no other changes of puberty or sex hormone effects occur, it is referred to as isolated premature thelarche, and needs no treatment.
There may well be two types of premature thelarche.
The classical type commences during the first year of life and tends to resolve by the age of 2.
There is a second form of premature thelarche, of which the age of onset is over 2 years of age and this tends to be more persistent and with a higher incidence of uterine bleeding.