Summarising ten years of research: new study explains why children enter puberty earlier

A new Danish study compiles ten years of research from one of the world’s largest and most detailed puberty cohorts and points to three main conclusions:

1. Puberty is occurring earlier
Compared with their mothers, Danish girls today get their first period at a younger age. Among boys, there are also signs of earlier pubertal onset than in previous Danish studies. The average shift is about three months and is large enough to potentially affect both wellbeing and later health.

2. Genes, pregnancy and family life all play a role
The timing of puberty is influenced, among other things, by the mother’s age at her own puberty, the mother’s health before and during pregnancy, maternal smoking during pregnancy, overweight and obesity in the child before puberty, and psychosocial and family-related stressors such as stress, divorce, absence of the father and sibling dynamics. Many of these factors are associated with earlier puberty - in some cases by as much as 4–5 months in both girls and boys.

3. We have confidence in the children’s own reports
The children assessed their pubertal stage themselves via online questionnaires. When the researchers compared these responses with clinical examinations and registry data, they corresponded well. This indicates that the self-reported information appears to be a valid measure of pubertal development.

These three conclusions are based on analyses from the Danish Puberty Cohort, in which nearly 16,000 children were followed closely through their pubertal development from 2012 to 2021.

A unique Danish puberty cohort

More than ten years ago, when Professor Cecilia Ramlau-Hansen from Aarhus University asked the question: “Are Danish children still entering puberty earlier - and why?”, no data existed that could provide a robust answer.

With support from, among others, the Independent Research Fund Denmark and AUFF Nova, she therefore established the Puberty Cohort - a large national cohort of nearly 16,000 children who have been followed closely through their pubertal development.

In the years that followed, she and a team of PhD students, Master’s students and postdocs validated the cohort and examined a wide range of possible causes of early puberty. A new review article brings together this entire research journey - “the crowning achievement”, as Cecilia Ramlau-Hansen herself puts it.

“It is quite special to be able to look back on more than ten years of work and piece everything together. It appears that the age of puberty continues to decline in Denmark today, and we have identified several concrete factors during pregnancy and childhood that seem to advance the timing of puberty,” says Cecilia Ramlau-Hansen.

“It is unique to have such detailed information”

The Puberty Cohort consists of 15,819 girls and boys born in 2000–2003. They are part of The Danish National Birth Cohort, where mothers and children have provided extensive information since pregnancy.

From 2012 to 2021, the children reported on their pubertal development themselves every six months between the ages of 11 and 18, for example breast development, pubic hair, menstruation, voice change and first ejaculation.

This gave the researchers a rare opportunity to combine knowledge of pubertal trajectories with information on everything from pregnancy conditions to daily life and psychosocial wellbeing in childhood.

“It is unique to have such detailed information both before and during puberty for so many children. This has given us an exceptional opportunity to identify factors and circumstances that can lead to earlier puberty,” says Cecilia Ramlau-Hansen.

Why is knowledge about early puberty important?

Early puberty involves far more than the timing of first menstruation in girls or voice breaking in boys. Research from the Puberty Cohort and other countries shows that early puberty can have consequences well into adulthood.

It is associated, among other things, with an increased risk of overweight and type 2 diabetes, hormone-related cancers, and poorer mental wellbeing during adolescence.

The results are therefore relevant for health professionals, researchers and policymakers working with children’s health and development.

“When biological and social factors early in life influence pubertal timing, puberty also becomes a signal of children’s overall living conditions. This is knowledge that can be used in prevention, counselling, and in identifying children who may need additional attention,” says Professor Cecilia Ramlau-Hansen, adding:

“Some of the risk factors for earlier pubertal development that we have identified cannot be changed, for example genetic predisposition. But other risk factors, such as childhood overweight and smoking during pregnancy, are areas where intervention is possible. This gives hope that the trend towards ever-earlier puberty can be slowed.”

Next chapter: from causes to consequences

The article in the International Journal of Epidemiology summarises the first ten years of research in the Puberty Cohort. In that time, the researchers have established the cohort, validated the data and investigated the causes of earlier puberty. But the research journey continues:

The participants are now young adults and are being followed further through national registers and new questionnaires. This makes it possible to investigate the significance of pubertal timing, for example, physical and mental health and for fertility and reproduction in adulthood.

“We have learnt a great deal about why children today enter puberty earlier. But we are only just beginning to understand what the timing of puberty means for adult life. The next important step is to examine the association with mental wellbeing and fertility later in life, and we are already working on that,” says Cecilia Ramlau-Hansen.

The continued work is supported, among other sources, by an Ascending Investigator grant from the Lundbeck Foundation and an ERC Synergy Grant from the EU.

Behind the research findings

• The study is a review article based on the Puberty Cohort, a sub-cohort of the Danish National Birth Cohort.
• The main finding is that the age of puberty continues to decline among Danish girls and boys, and that several biological and psychosocial factors can advance the timing by up to 4–5 months, for example maternal age, smoking and family-related stressors.
• The study was conducted by researchers at Aarhus University: Anne Gaml-Sørensen, Andreas Ernst, Nis Brix, Lea Lykke Harrits Lunddorf, Anne Hjorth Thomsen and Cecilia Ramlau-Hansen (PI).
• The project is supported by the Independent Research Fund Denmark (DFF), AUFF Nova and an ERC Synergy Grant (BIOSFER).
• The results are published in a peer-reviewed article in the International Journal of Epidemiology.
• Data originate from the Danish National Birth Cohort (DNBC). Researchers may apply for access through the DNBC data access procedure.
• Read more in the scientific article: “Ten-year findings from the Puberty Cohort: a sub-cohort within the Danish National Birth Cohort”

Contact

Professor Cecilia Ramlau-Hansen
Aarhus University, Department of Public Health
Telephone: +45 26 29 57 15
Email: chrh@ph.au.dk