A new study on the age of initial breast development as the first sign of puberty in girls shows that over the last 40 years, this has come down by an average of 3 months per decade. The study is published in the journal Pediatrics.
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Puberty is a landmark in human development, with significant and noticeable changes both of the mind and the body. The process is dependent on the awakening of the dormant hypothalamo-pituitary-gonadal axis, at which point the hypothalamus regulates the production of male and female reproductive hormones and feedback loops which causes the individual to become sexually mature and capable of reproduction ultimately.
A variety of factors can affect this reawakening, however, ranging from genetic differences through malnutrition or overweight, to stress. The result of these influences could be a change in the age of puberty.
The most common way of assessing female age at puberty is by asking about the age of the first menstrual period (or menarche). However, this may lead to wrong data as it depends on the memory of the woman, often many years later. Moreover, it is an event that occurs late in the sequence of pubertal events.
For these reasons, the current study was based on thelarche, the first and most accurate sign of breast development. This is the most important clinical sign that puberty has begun. The Tanner scale is an accepted clinical scale used to evaluate breast development.
The study was carried out using 30 papers from 1997 to 2013. The researchers looked at all studies that used a clinical method (inspection and palpation) to assess breast development in healthy girls, and that had good statistical quality. They specifically examined the age at thelarche in these girls. They deliberately excluded studies in which most girls were malnourished or obese, or had certain diseases, to avoid the confounding effect of these factors on the age of onset of puberty.
The analysis showed that the median age at which healthy girls attained Tanner breast stage 2 was different based on race and geographical location.
- In Europe: 9.8 to 10.8 years
- In the US: 8.8 to 10.3 years
- In Africa: 10.1 to 13.2 years
- In Asia: 8.9 to 11.5 years
- In the Middle East: 9.7 to 10.3 years
When the studies were analyzed by date, the age at which the girls began to develop Tanner breast stage 2 went down by 0.24 years, which is almost 3 months for each of the 10-year periods within the study. Region-wise trends were not statistically significant as the number of cases per region was too low to give the study sufficient power.
This is the first study ever to examine thelarche rather than menarche as a measure of the age of onset of puberty. It shows that this is declining worldwide at a rate of 3 months per 10 years.
Importance of earlier thelarche
The reason why this is important is that the definition of normal puberty depends to a great extent on the ‘normal’ age of onset of puberty. Secondly, the stage of breast development is vital in differentiating normal puberty from precocious puberty. If this is truly going down, it is essential to have new definitions or cut-offs in place to avoid the inappropriate diagnosis of a healthy girl for precocious puberty.
In one study in Finland, girls with early breast development made up over a third of girls with precocious puberty. These girls might have been wrongly investigated for hormonal or other abnormalities through their physical development is rightly following the lead of the precocious breast development.
For instance, magnetic resonance imaging of the brain is often performed in girls who show the onset of puberty before the age of 8 years, along with blood tests to look for hormonal levels. In light of the current findings, investigating all pubertal girls below 8 years would mean unnecessarily subjecting a lot of normal girls to such evaluation.
Thus, this study could initiate a process of redefining traditional concepts of precocious puberty to recognize healthy development in a larger cohort of girls.
Evidence of hormonal changes due to endocrine disrupters
Another implication of early pubertal age could be the evidence it offers that environmental changes are affecting human health. Early menarche is known to have a positive relationship with body weight, which is also linked to earlier thelarche. The global trend towards higher obesity rates could thus explain at least part of the observed early thelarche pattern.
Another factor could be the exposure of modern girls to a host of chemicals that influence the levels and the actions of the gonadal steroids such as estrogen and testosterone. These are therefore called endocrine disrupters. They could either increase the hormone activity or oppose it, increase the synthesis or break them down. One well-known example is methylparaben; another is the persistent pesticide DDT.
The challenge is enormous, as the number of possible endocrine-influencing chemicals found in the modern environment runs into the thousands. Moreover, it is to be feared that many may work in synergy with each other to produce a net effect that is far more pronounced than that of each chemical singly, even at levels of individual agents that are not enough to have biological activity. This is called the cocktail effect.
Early puberty and long-term health
Thirdly, the early onset of puberty is associated with poor long-term health outcomes in many women. For instance, early menarche can increase the risk of later obesity and cardiovascular disease, which lends urgency to the task of defining this condition, and if possible, preventing its harmful effects.
The study has some limitations, such as the inclusion of several small studies (which lack sufficient statistical power), and the inability to account for body mass due to the absence of such information in many studies, and the apparent differences in socioeconomic background of the many study groups. However, the finding of earlier thelarche holds good the world over, with the earliest age of onset being in the USA and the latest in Africa. Researcher Alexander Busch says, “Fighting childhood obesity and avoiding excessive exposure to environmental chemicals could help to avoid early pubertal onset.”
Some experts do disagree, saying that the conclusion could also be that the whole process of puberty is being stretched out rather than an actual shift in the age of menarche. It is not clear how the increased duration of hormonal activity in the former case would not be equally harmful, however.
Eckert-Lind C, Busch AS, Petersen JH, et al. Worldwide Secular Trends in Age at Pubertal Onset Assessed by Breast Development Among Girls: A Systematic Review and Meta-analysis. JAMA Pediatr. Published online February 10, 2020. doi:10.1001/jamapediatrics.2019.5881