Risk of multiple sclerosis inversely associated with birth order position.

The aim of this study was to ascertain whether there is an association between risk of multiple sclerosis (MS) and birth order position. Our reference population was 198,000 persons born in the period 1930-50 and recorded in the register of school health records from the school health service of the Copenhagen council. We compared 46 persons from the register who had developed MS with matched controls from the register, three for each case. An inverse association between risk of MS and birth order position was found. Early birth orders tend to delay exposure to an infectous agent from early childhood to a later age. Therefore, our finding supports the hypothesis that MS is causally related to an infection that is inapparent when it occurs in early childhood, while infection later in life may result in severe disease