Girls are growing up too fast nowadays is a phrase we increasingly hear. And now mounting scientific research suggests it may literally be true. Studies from a variety of different industrialised countries are finding that girls in particular are starting and often reaching puberty earlier than they used to.
Why does this matter to schools? Because research shows that girls who reach puberty early are more likely to get into trouble at school - including greater absenteeism and truancy - and have lower grades.
Early puberty can change a girl's life trajectory. There are also conflicting estimates as to exactly how much earlier this precocious puberty is occurring.
A study published in the British Medical Journal reported that the average age of menarche (age at first period) in British teenagers is 12 years and 11 months. Previous surveys in the Fifties and Sixties found that the average age was about 13 and a half. More than 1,000 girls aged 12 to 16 in schools across 10 British towns took part in the survey. Almost one girl in eight reaches menarche while still at primary school. (Whincup et al, 2001)
While menstruation may mark the end point of puberty and the arrival of womanhood, breast development is the most reliable milestone used to mark the beginning of puberty and many believe it is this which continues to occur earlier.
One American study of 17,077 girls between ages three and 12 found that Caucasian girls are developing breasts and pubic hair about a year earlier than previously thought, with African-American girls developing two years earlier. Nearly half (48.3 per cent) of African-American girls and 14.7 per cent of Caucasian girls had begun development by the age of eight. The average age is 8.9 years for African-American girls and 10.0 years for Caucasian girls. They concluded that early puberty is "a real phenomenon". (Herman-Giddens, M.E. et al, 1997)
It is accepted that the normal age for a girl to begin to develop the first signs of puberty is 10 and above. Now, according to researchers, some enter puberty as young as seven, and Sweden's Karolinska Institute has co-ordinated 12 European research teams to look more closely at the age of onset and possible causes of early puberty. Recent American research points to one contemporary suspect: increasing body fat. Girls who are obese at the age of four are significantly more likely to start puberty early.
A team of pediatric endocrinologists at the University of Michigan conducted regular laboratory physical examinations of 354 girls aged three to 12 and cross-checked this with reports from the girls' mothers.
They found that for each one-point increase in body mass index at 36 months, the odds of having earlier puberty at age nine increased by 44 per cent. And of the heaviest 15 per cent of girls, 50 per cent had started developing breasts by the age of nine. The report concluded: "Increased body fatness precedes the onset of puberty in healthy girls". (Lee et al, 2007)
Now British health professionals are concerned that puberty will arrive earlier in Britain as levels of child obesity continue to rise.
Early puberty is more than an inconvenient surprise for children and families because it is linked with increased risk for a variety of problems. Teachers have a vested interest too as it can change a girl's journey through life.
A recent wide-ranging review of scientific studies concluded that while late puberty "has been associated with higher grades. Early maturing girls are more likely to exhibit poor academic performance in high school than on-time or later maturing peers. These adolescent trends in achievement generate differences in career and income level that persist through adulthood."
For example, in a sample of girls who reached puberty before the age of 11, only 2 per cent pursued higher education, irrespective of their intelligence or parent's level of education. However, the fact that they had an older than usual social network turned out to be an important factor.
The reason seems to lie in the link between early puberty and problem behaviour. Girls who reach puberty early are more likely to get into trouble at school and display more absenteeism and truancy.
They are far more likely to have underage sex and bear children before their peers, and to report that subsequent difficulties with childcare will affect their pursuit of higher education. Consequently, they tend to be employed in lower-paying, less prestigious jobs. (Mendle et al, 2007)
Sudden unexpected physical and emotional changes - from weight gain and mood changes to acne - can be extremely upsetting and girls find them difficult to deal with.
There can be a more serious side to these changes. A growing body of research is finding that girls maturing earlier are more likely to become depressed, delinquent, aggressive, socially withdrawn and suffer sleep problems, start drinking, smoking, or drug abuse, have lower self-esteem and suicide attempts. (Bratberg et al 2005; Mrug et al 2008)
Young children who look older may tend to hang around with older children and may feel a need to prove themselves by acting older. Yet these are still young children whose judgment is probably not as mature as their appearance.
Younger girls undergoing early puberty need extra guidance, supervision and support from parents and teachers
Dr Aric Sigman is a fellow of the Royal Society of Medicine, member of The Institute Of Biology and associate fellow of the British Psychological Society
Bratberg, G. H. (2005) Sexual maturation in early adolescence and alcohol drinking and cigarette smoking in late adolescence: A prospective study of 2,129 Norwegian girls and boys, European Journal of Pediatrics, 164 (10):621-5
Herman-Giddens, M.E., et al (1997) Secondary Sexual Characteristics and Menses in Young Girls Seen in Office Practice: A study from the Pediatric Research in Office Settings Network, Pediatrics, 99, 505-12
Lee, J. M., et al (2007) Weight Status in Young Girls and the Onset of Puberty, Pediatrics, 119:3, 624-630
Mendle, J., et al (2007) Detrimental psychological outcomes associated with early pubertal timing in adolescent girls, Developmental Review, 27, 151-171
Mrug, S., et al (2008) Positive parenting and early puberty in girls: Protective effects against aggressive behavior, Archives of Pediatrics amp; Adolescent Medicine, 162:8, 781-6
Whincup, P. H., et al (2001) Age of menarche in contemporary British teenagers: Survey of girls born between 1982 and 1986, British Medical Journal, 322, 1095-1096.