"Girls who are tall and lean in childhood are more at risk of later developing endometriosis, research has found", The Guardian reports.
Endometriosis is a condition where cells or tissue that normally lines the womb develops outside of the womb, elsewhere in the pelvis, such as on the ovaries. This can cause a range of symptoms, including pelvic pain, increased period pain, and difficulties in getting pregnant. Treatments for endometriosis include painkillers and hormonal treatments, such as the combined contraceptive pill.
A new Danish study reviewed the school health records for more than 170,000 girls who had their weight and height measured at ages 7 to 13. During follow-up, around 1 in 100 developed endometriosis. The researchers found that increased child height was linked with 9% increased risk of endometriosis.
People should not be overly concerned by these findings. The study does not prove that being tall during childhood triggers endometriosis in later life. Also, the participants' baseline risk in this study was around 1.25%. A 9% increase on this only increases it to a 1.37% risk. So an individual's risk is likely to remain low, whatever their height.
It's an interesting finding, but it does not explain the causes of endometriosis, which are not clearly understood. However, the findings could possibly provide clues which may be followed up in the future.
Where did the story come from?
The reviews were carried out by researchers from Bispebjerg and Frederiksberg Hospital and Copenhagen University Hospital Rigshospitalet, both in Copenhagen, Denmark, and the National Cancer Institute, Bethesda, US. The study was funded by Dr Sofus Carl Emil Friis and Olga Doris' Fund and the intramural research programme of the US National Cancer Institute. It was published in the peer-reviewed scientific journal Annals of Human Biology.
The Guardian's coverage could benefit from highlighting the small baseline risk of endometriosis for any individual female, and that this study does not explain the causes of endometriosis.
What kind of research was this?
This was a cohort study using data collected from Danish school health records and medical records to look at the link between a female child's height and body mass index (BMI) and endometriosis.
Studies such as this are useful for looking at links between different exposures and health outcomes, particularly in a population-based cohort such as this which use a large quantity of data gathered over a long period of time. However, they cannot prove cause and effect as other factors may be involved.
What did the research involve?
The study used The Copenhagen School Health Records Register which contains information on the school health examinations for nearly all children born 1930 to 1996.
Examinations were done between ages 7 and 13 years. From 1968 onwards, all Danish residents, including children, were given a personal identification number, which could then be linked with the national health register.
From 1977 onwards, the national patient register had documented all hospital admissions for endometriosis. From the mid-1990s they also documented outpatient hospital visits.
This study included 171,447 women who had their height and weight recorded at school, and with follow-up from 1977.
BMI, height and weight were compared by looking at z-scores. This is a scoring system based on standard deviation, which means how much the child's measurements deviate from the average for their age and sex.
What were the basic results?
During follow-up, 2149 women developed endometriosis (1.25% of the cohort). The average age at diagnosis was 37 years.
Increased height was linked with increased risk of endometriosis. Each increase in height z-score at age 7 years was linked with 9% increased risk (hazard ratio [HR] 1.09, 95% confidence interval [CI] 1.05 to 1.14).
Meanwhile, higher BMI in childhood was linked with decreased risk of endometriosis. Each increase in BMI z-score at age 7 years was linked with an 8% decreased risk (HR 0.92, 95% CI 0.88 to 0.96).
How did the researchers interpret the results?
The researchers conclude: "Lean and tall girls are more often diagnosed with endometriosis… These findings suggest that indicators of endometriosis risk are already apparent at early ages."
Endometriosis can have considerable effects on quality of life and can be difficult to manage. The causes are not fully understood, however possible causes include genetics, problems with the immune system, or some of the uterus lining may flow backwards during a monthly period (retrograde menstruation). It may be caused by several factors, including factors which have not yet been detected.
This study will be of interest to specialists in the field, but it will not provide an answer for endometriosis. Observational research such as this cannot prove direct cause and effect, particularly when we do not understand all the possible health and lifestyle causes. Therefore such factors cannot be taken into account in the analysis.
The increased risk from being taller (or having a lower BMI) was also very small. Only 1.25% of this sample developed endometriosis. A 9% increase on this small baseline risk only increases the risk to 1.37%. Therefore any individual's risk appears to remain low, whatever their height or weight.
Overall, even if there is a direct link with height, these findings will not currently help in the prevention (if this is possible) or effective management of the condition.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 10 March 2020
Independent, 10 March 2020
Links to the science
Annals of Human Biology. Published online 9 March 2020