Children are growing up so fast these days that they barely seem to have been touched by childhood before puberty begins. They are tending to behave precociously, using sexual language and sometimes dressing older than their years, as a result of being immersed in an increasingly sexualised society. In the US, entering puberty at the age of eight is now considered normal, particularly in the Black and Hispanic communities. The world's youngest known mother was a five-year-old Peruvian who gave birth by Caesarean section to a six-pound baby.
At Tallaght Hospital in Dublin, Prof Hilary Hoey, a paediatric endocrinologist, has seen girls as young as two and three who were menstruating as a result of "precocious puberty". Normally, Irish girls may begin to menstruate, and thus become fertile, anytime between the ages of 10 and 15. They begin to show the first signs of puberty, such as breast buds and sometimes pubic hair which may be accompanied by acne, as early as age nine. A study by Prof Hoey, a paediatric endocrinologist at Tallaght Hospital, found that Irish girls have the latest menarche in Europe: age 13-and-a-half on average, compared to age 13 in England, age 12-and-a-half in the US and age 11-and-a-half in Italy.
But for some, hormones kickstart much earlier and for parents and children alike this can be a confusing and worrying development. No sooner have the toddler tantrums been banished, than hormone-linked mood swings become an issue. But when is this early development normal, and when is it a case of "precocious puberty", where a little girl who still plays with dolls becomes trapped inside the body of a fully fertile woman?
"If you have a child of seven or eight who is in a class of her chronological peers but has developed sexual urges and curiosities, she can feel isolated, can get into trouble and can be particularly vulnerable," comments Prof Joe McKenna of St Vincent's Hospital, Dublin, who is an endocrinologist with a special interest in the reproductive system.
Puberty is a complex developmental process that culminates in sexual maturity. It is governed by the hypothalamic-pituitary-gonadal axis, a delicate hormonal system connecting the brain to the reproductive system that, when triggered, results in the appearance of secondary sexual characteristics, acceleration of growth and, ultimately, the capacity for fertility.
While girls go through visible changes, for boys the arrival of puberty is less precise because the most obvious signs - increased testicular size, scrotal thinning, pubic hair and penile enlargement - are hidden from view, but most will begin maturing at about the age of 11-12.
When girls younger than nine and boys younger than 10 experience pubertal changes, parents may ignore the problem out of fear that by drawing attention to the child's emerging sexuality they will make their children feel self-conscious. However, endocrinologists are agreed that for both psychological and medical reasons, such children should be assessed for "precocious puberty" - a condition 10 times more common in girls than in boys.
With girls, eight out of 10 cases of precocious puberty are benign or "idiopathic" - in other words, the hormones have switched on early for an unknown reason. The hormone concerned is gonadotropin-releasing hormone (GnRH), which is secreted from the hypothalamus.
Boys are more likely to have a pathological cause for this early switching-on of the maturation process. There is a rare inherited form of precocious puberty that affects the central nervous system and affects only boys.
When a child is being assessed, the endocrinologist will order CT scans of the brain, since any abnormality (due to a tumour or trauma) can result in the loss of sensitivity to inhibin, a chemical which prevents GnRH from being secreted until the onset of normal puberty. Blood tests are aimed at looking for gland problems that cause abnormal hormone levels, especially very low levels of thyroid hormones or high levels of adrenal hormones. An X-ray of the wrist determines bone growth, and index of physical maturity which may excede the child's chronological age.
In most cases of precocious puberty, the only major concern is the child's growth. Children who have early puberty experience a rapid growth spurt which temporarily turns them into giants amongst their peers. But because their puberty comes sooner, these children stop growing earlier and end up shorter. While they may appear tall for a year or two, in the long run they become dwarfed by their peers, who outstretch them as they grow through normal puberty.
Prof James Devlin, a retired endocrinologist with Beaumont Hospital who has seen many cases of precocious puberty, once had as patients twin girls, where one of the twins had precocious puberty and suddenly shot past her sister in height, yet ended up shorter than her sister. He advises parents, if they are concerned that their children are developing early, to take the oldfashioned measure of recording children's heights on the wall.
An early puberty does not always affect height. Many girls (particularly those who begin puberty after their seventh birthday), will start puberty early, but still go through each of the stages at a more typical pace. While their "adolescent" growth spurts are over early, they will continue to grow until their bones reach final maturity at about age 16.
From a psychological point of view, sexual feelings can be confusing for a child who is not yet emotionally mature enough to handle them - "boys can become very aggressive and frustrated," remarks Prof Hoey - while a seven-year-old girl trapped inside a woman's body may have serious problems with her self-image and in her relationships with other children.
A Swedish study found that girls who begin menstruating younger than age 11 are at higher risk of teenage pregnancy, which makes sense when you consider that such girls have women's bodies combined with childish emotions.
Taking the physical and psychological aspects into account, the decision which parents have to make, advised by a paediatric endocrinologist, is whether to treat the child with hormones in order to delay puberty. The most effective treatment is a horomone (GnRH agonist) which normally comes from the hypothalmus in pulses. If, however, it is given continuously by injection, it has the opposite effect and suppresses the production of the pituitary hormones, thus turning off the hypothalmic pituitary gonadal axis and stopping the progression of puberty. The hormone is usually given by injection every three or four weeks.
However, the decision on hormone treatment may not be clear-cut. If a girl, for example, appears to be growing to a normal height and is able to cope with the psychological aspects of maturing early, it may be best to allow her body to continue on its own early schedule. As Prof Hoey points out, the hormone treatment used "appears" to be safe, but it is conceivable that in 20 years' time a negative side-effect may be discovered. By that time, the girl will be in her 20s and probably about to start her reproductive life.
Talking honestly with the girl about her growth and maturation and taking her own views into account is important, Prof Hoey believes.
"It's very important that the child understands what is happening and that parents explain and talk to them," she advises. "The child needs to know that puberty is a normal process and that they are not abnormal."