thumb|Tanner scale of female breast development
Thelarche, also known as breast budding, is the onset of secondary breast development, often representing the beginning of pubertal development. It is the stage at which male and female breasts differentiate due to variance in hormone levels; however, some males have a condition in which they develop breasts, which is called gynecomastia. Thelarche typically occurs between the ages of 8 and 13 years with significant variation between individuals. Moreover, puberty is considered delayed if breast development does not start at age 13 or if a female has not had her first period (menarche) within three years of thelarche. Additionally, secondary breast development occurring before the age of 7 years could be a sign of premature thelarche or precocious puberty. Of note, for some girls, thelarche will occur, with subsequent regression of breast development, and then months or years later, normal breast growth will commence again accompanied by normal pubertal changes; this is termed transient thelarche. This system does not use breast size but instead examines the shape of breasts, nipples, and areolae to determine the progression of growth. Other contemporary trends that may be contributing to this change in the onset of thelarche include increasing body mass index (BMI), changes in pubertal timing, and environmental exposures. Studies also indicate associations between the average age of thelarche and race. Due to the decreasing age of thelarche over time, there is discussion of adjusting the cutoff for early thelarche to 7 years old for white females and six years old for black females. Moreover, the prevalence of premature thelarche has been increasing over the past several years which many attribute to pesticides. Experimental data indicate the impact of chemical contaminants in gestation and puberty, with the chemical such as phthalates having a high association with premature thelarche frequency.
Premature thelarche
thumb|Infant with premature thelarche
Premature thelarche is a benign condition in which a young female has breast development before age eight without any accompanied pubertal changes. Individuals undergoing isolated premature thelarche do not experience menstruation, pubic hair growth (pubarche), or the bone growth characteristic of puberty. The breasts of these females typically exhibit alternating progression and regression patterns of growth in 6-week intervals, often completely decreasing in size within 1.5 years. Due to the benign nature of the condition and the tendency to self-resolve, premature thelarche does not require treatment. Though premature thelarche and precocious puberty are distinct conditions, there is some overlap; an estimated 14 to 18 percent of females who exhibit premature thelarche will additionally develop precocious puberty.