Research published in the latest online edition of The Lancet Neurology says that the fact that first-borns have a higher chance of having multiple sclerosis is nothing but a myth. Birth order of the children does not increase their risk of having the disease, nor does their family size have any affect on the incidence of multiple sclerosis.
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.
Researchers from University of British Columbia, Canada, had analyzed data from over 10900 individuals with MS and over 26300 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.
Researchers had said that their 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 suggest that environmental risks for MS must be accounted for by factors that do not affect birth-order position.
Source: Newswise, The Lancet