Girls who are tall and lean in childhood are more at risk of later developing endometriosis, research has found.

Endometriosis is a painful, often debilitating, condition in which tissue similar to that found lining the womb is found elsewhere in the body, such as the bowel or ovaries. When it breaks down and bleeds, as it would in the womb, it can cause inflammation and pain. It is believed that up to 10% of women live with the condition, with some left infertile as a result.

Now researchers have studied decades of data to reveal the condition is more common among women who were tall and lean when young, a finding that may help identify those at greater risk.

“Body size during these ages is an indicator for later risk,” said Dr Jennifer Baker, a co-author of the research from the University of Copenhagen and Frederiksberg hospital in Denmark. “It really tells us that the roots of this disease lie earlier in life than people have previously thought,” she said.

Writing in the Annals of Human Biology the researchers reported that they analysed data from more than 170,000 women born in Denmark between 1930 and 1996. Danish school-based health programmes meant all children had their height and weight measured between the ages of seven and 13 years.

The team also looked at hospital data, collected since 1977, to track until mid-2017 records of endometriosis or adenomyosis – a condition in which tissues akin to the lining of the womb are found in its muscular walls – in those aged 15 or older.

Overall 2,149 women were diagnosed with endometriosis, and 1,410 with adenomyosis.

The results reveal a lower childhood body mass index (BMI) is linked to a greater risk of being diagnosed with endometriosis. More specifically, for seven-year-old girls of average height, a child of average weight has an 8% lower risk of endometriosis than a child who weighed 2.3kg less. “By 13 [years old] for two girls with an average height, the one who weighs 6.8kg less has a 6% increased risk,” added Baker.

The study also reveals that being taller as a child is linked to a greater risk of endometriosis – chiming with previous research in adults.

The researchers note their findings are similar regardless of the birth year of those involved, suggesting the associations are down to a biological mechanism.

However, the team found little evidence of links to either BMI or height when it came to adenomyosis.

The authors say one possible explanation for the relationship between endometriosis and height is that both may be linked to oestrogen levels: the hormone is known to promote growth of the lining of the womb and is believed to play a role in growth spurts during puberty.

However, the relationship between endometriosis risk and BMI is more puzzling: a higher BMI, and greater weight, has previously been linked to a greater production of oestrogen – yet the latest study suggests a higher BMI is protective against endometriosis.

“It is a very complex association and it may not be same pathway between BMI and height,” said Baker.

The study has limitations, including that most participants were white, while methods for diagnosis and awareness have changed over the years. What’s more, endometriosis is thought to be underdiagnosed, and the current study relies on hospital records – meaning the findings may only apply to severe cases.

Krina Zondervan, professor of reproductive and genomic epidemiology at the University of Oxford, said other research had suggested the apparent relationship between BMI and endometriosis might actually be down to how fat was distributed around the body.

“Fat distribution differences are strongly hormonally regulated, as is risk of endometriosis,” she said. But, Zondervan stressed, the link to height remained unclear, since the new study found such an association before the children would have been expected to begin puberty.

“Childhood height is more influenced by nutritional as well as genetic factors [than oestrogen],” she said. “More work needs to be done to understand this link.”