The number of younger siblings you have may affect your risk of developing a brain tumour. Risk is highest for those with many younger siblings. This is shown in a new study from the Center for Family and Community Medicine, a cooperative venture of the Karolinska Institutet and the Stockholm County Council.

“Relatively little research has been done on the causes of brain tumours, particularly on whether family relationships can affect risk”, says researcher Andrea Altieri. “The results of the few studies that have been done are inconsistent and the sample sizes have been small, so we wanted to learn more.”

The study is the largest to date on whether number of siblings and relative birth order can affect the risk of brain tumour. It shows that children and adolescents under age 15, who have three or more younger siblings, have two to four times higher risk of certain types of brain tumours than same-aged children without siblings. However, no obvious raised risk was found in people over age 15 with many siblings. Dr. Altieri emphasizes that brain tumours and cancer of the nervous system are rare:

“About 80 children and 1,200 adults are affected in Sweden each year this out of a population of 9 million”, he says.

Research is based on information from the years 1958-2002 on approximately 13,600 cases of brain tumour. The information was obtained from various registers that include data on all Swedes born 1932 and after, plus their parents. Researchers took age, sex, profession, geographical region of residence, and whether anyone else in the family had cancer into account.

Dr. Altieri cautions that conclusions must be drawn with care. However, he believes the results hint that the environment in families with many children plays a role:

“There might be an association with infections. Children with many siblings probably come into contact with lots of other children outside the family and are exposed to a wider range of infections than children without so many siblings”, Dr. Altieri says. He also notes that it´s necessary to complement this kind of register-based study with other kinds of research to gather more facts, for example, laboratory studies that focus on the connection between suspected viruses and tumours.

The director of research for the study is foreign adjunct professor Kari Hemminki of Karolinska Institutet. The study is published in Neurology.