Birth order does not affect risk of multiple sclerosis

08/19/05

EMBARGO: 00:01H (London time) Monday August 22, 2005. In North America the embargo lifts at 6:30pm ET Sunday August 21, 2005.

Older siblings, who have an early birth-order position, do not have a higher risk of multiple sclerosis (MS) than their younger siblings, according to a study published online today (Monday August 22, 2005) by THE LANCET NEUROLOGY.

Studies suggest that both environmental and genetic factors have a role in MS. Previous research has reported a higher risk of MS in offspring of small families and those with early birth-order position. This has been explained in context of the hygiene hypothesis, where children who are exposed to infections introduced by older siblings are at a reduced risk of developing MS. However, until now, studies that have looked at birth order and MS risk have been small and therefore less reliable.

Dessa Sadovnick (University of British Columbia, Canada), George Ebers (Oxford University, UK), and colleagues analysed data from over 10, 900 individuals with MS and over 26, 300 healthy siblings. They looked at the data in two groups; sibships where 1 sibling had MS, and sibships in which at least 2 siblings had MS. Using statistical techniques the researchers calculated the expected birth-order, controlling for sibship size, and compared this to the actual observed birth order. Overall, they found no link between MS risk and birth order position. In fact, for larger sibships, individuals with MS tended to be born later in birth order position than expected, contrary to the prediction of the hygiene hypothesis.

Professor Sadovnick states: "This study does not support the prediction of the hygiene hypothesis, which suggests that people with MS would be born earlier than expected within their sibships…The data presented here cast no doubt on the importance of environmental factors to MS risk, and suggest that environmental risks for MS must be accounted for by factors that do not affect birth-order position."

Source: Eurekalert & others

Last reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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