Birth order not a factor in multiple sclerosis risk

A new Canadian study suggests that the youngest children in a family are not less likely than older siblings to develop multiple sclerosis (MS).

This is contrary to the "hygiene hypothesis" whereby being too clean is not a good thing.

According to this theory, infections at an early age train the immune system to respond appropriately to the environment, and this supposedly protects children from certain diseases such as asthma and MS, so having older brothers and sisters should raise the odds of such infections, and therefore reduce the risk of MS.

In order to explore this hypothesis, Dr. A. Dessa Sadovnick, from the University of British Columbia in Vancouver, Canada, and colleagues analyzed data from 10,995 MS patients and 26,336 healthy siblings drawn from a large population-based registry.

The researchers found that late birth order did not cut the risk of MS, and in fact, in large families with only one affected sibling, there was some evidence that late birth order actually raised the risk, as affected siblings were slightly younger than unaffected siblings.

The team concluded that their study does not support the prediction of the hygiene hypothesis.

Published in Lancet Neurology, online August 22, 2005.