In girls, the ovaries begin to produce female hormones such as estrogen. The first sign of puberty can then appear in the obvious form of mammary gland growth. This usually happens between the ages of 8 and 13. The appearance of hair in the pubic area and under the arms as well as a peak in growth quickly follows this stage. Finally, the first menstrual period, called menarche, occurs. This usually occurs about two and a half years after the onset of puberty. Complete development of breasts marks the end of this process during which the young girl’s pelvis becomes enlarged, her adipose tissue develops and is distributed differently in the body and her hips and buttocks become rounder. Regular ovulatory cycles however may not be seen before several years after menarche.

In boys, the first sign of puberty occurs with an increase in testicular volume, which is almost imperceptible to the individual and his family circle. This is in contrast to the development of breasts in girls. The testicles begin to produce testosterone and the appearance of pubic hair then follows, usually between the ages of 10 and 14. While the increase in testicular volume further progresses, the penis also becomes larger (between the ages of 11 to 12 years on average). As in the case of girls, the end of puberty, including the full development of pubic hair, only occurs several years after the first signs of puberty appear. Boys are different to girls in that the peak in growth is later during puberty (see the sixth-year primary-school class photo) accompanied by a change in the voice. They are now fertile, but certain changes in hairiness (face and/or chest) and muscle development can continue to increase thereafter. The last “detail” for both sexes: all these transformations can be accompanied by the development of acne, the appearance of body odor and mood changes.

Far from being a bed of roses

As we have seen, puberty follows a well-defined pattern of changes with these changes being correlated to different stages. We could therefore imagine that this process is a long one and though it is not necessarily a bed of roses, it is a relatively similar phenomenon in adolescents. Nothing could be further from the truth: large differences may occur, from the moment when this phenomenon appears, for the length of its duration to the time when it ends.  

For a long time, the attention of researchers was focussed on girls and the increased age at which menarche occurs, the authors of "Changes in Pubertal Timing: Past Views, Recast Issues" remind us. In fact the scientific literature on the subject was largely preoccupied with an early onset of menarche. The phenomenon has been observed since the mid-19th century in many European and North-American countries, that is to say, in the so-called “developed” countries. According to observations, in one century, from 1850 to 1950, the first menstruations occurred four years earlier on average. "This represents a spectacular increase and has been interpreted as the consequence of an improvement in dietary hygiene”, details Professor Bourguignon. However, following this line of logic, the first menstrual periods could have been expected to occur further earlier during the past fifty years among a majority of young girls. Moreover, projected figures predicted an average age of menarche of more or less 12 years of age at the end of the 20th century but this theory has not really been confirmed…

Slower than expected...

After the sixties, the advance of pubertal maturity slowed down and/or its decline stopped in a certain number of countries. During the last few years, the age of menarche has become relatively stable in countries such as Belgium. “For example”, continues Professor Bourguignon, “A study conducted in Brussels in 1960 fixed the age of first menstruations at 13. Currently, we are still quite close to that age”.  Likewise, recent publications show that there is an increase in the average age of menarche – this is the case in Denmark or in Holland -, but it remains moderate.

So, has the time at which puberty arrives stopped changing? If the answer is yes, what is the reason? In fact, as Professor Bourguignon explains, “what we are noticing, is that the onset of puberty, with the development of mammary glands continues to concern more girls at an early age and even at a very early age (before 8 years of age)”. Yet despite this earlier onset of puberty, the age at which first menstruations are occurring remains relatively unchanged or only moderately so.  

The researchers point out that boys too are not unaffected by these changes. It has been observed that for them, the final stages of puberty can occur later. “These changes show that, overall, the pubertal process is getting longer”, explains Professor Bourguignon.

This observation is certainly not insignificant: this lengthening of the pubertal process just like the heterogeneity of pubertal events, has led to revision of knowledge about the role and place of the different indicators and the mechanisms involved in pubertal onset. For example, the variations in the age of puberty can no longer be considered as the result of a complete separation of environmental and genetic determining factors: each of these plays a role, but associated factors also appear and these factors date from fetal life.