Month of birth effect is real

Month of birth effect is back on the MS agenda. #MSBlog #MSResearch

"Month of birth effect in MS is back on the agenda, big time. I don't think the Stephen Sawcer paper said it did not exist all they were saying is that there are potential confounders that need to be controlled for. This paper below does just that and demonstrates at a country level that more MSers are born in April than expected. I can't stress how important this finding is for MS prevention strategies and now needs to be confirmed in other countries."

"What this paper is saying that some environmental factor is probably acting in utero (inside the womb), and/or in early childhood, increase the risk of MS. Based on other findings this is likely to be low vitamin D levels. The current hypothesis is that if the immune system matures in a vitamin D deficient environment it increases one's chances of getting MS in later life. This means that vitamin D prevention studies may need to start in utero. This is why we have been recommending that all our patients with MS, who are planning to fall pregnant, make sure they are vitamin D replete before falling pregnant and whilst they are pregnant."

"What is needed however is a randomised international study of vitamin D supplementation during pregnancy to see if it reduces autoimmune disease in later life. Yes, autoimmune disease in general; the month of birth effect is not unique to MS and has been noted with other autoimmune diseases as well."


Torkildsen et al. Month of birth and risk of multiple sclerosis: confounding and adjustments. Ann Clin Transl Neurol. 2014 ;1(2):141-4.

Background: A month of birth effect on multiple sclerosis (MS) risk has been reported from different countries. Recent critics have suggested that this finding is caused by confounding and that adequately adjusting for year and place of birth would markedly reduce this effect. All inhabitants in Norway are registered in the Norwegian Population Registry (Statistics Norway), making this an ideal area for performing adjusted analyses. 


Methods: Using the entire Norwegian population born between 1930 and 1979 (n = 2,899,260), we calculated the excess between observed and expected number of births for each month for 6649 Norwegian MS patients, 5711 mothers, 5247 fathers, and 8956 unaffected siblings. The analyses were adjusted for year of birth and place of birth according to the 19 counties in Norway. 

Results: An unadjusted analysis revealed 13% fewer MS births than expected in February (P = 0.0015; Bonferroni corrected P = 0.018), 10% more in April (P = 0.0083; Bonferroni corrected P = 0.0996) and 15% more in December (P = 0.00058; Bonferroni corrected P = 0.007). Adjustments for both year and place of birth significantly altered our results for February and December, but even after these adjustments there were still 10% more MS births than expected in April (P = 0.00796; Bonferroni corrected P = 0.096). 

Conclusions: MS patients had a higher incidence of April births than their siblings (Fisher-exact test; P = 0.011), mothers (Fisher-exact test; P = 0.004), and fathers (Fisher-exact test; P = 0.011) without MS. Adjustments for confounding significantly affected our results. However, even after adjustments, there appears to be a persistent higher than expected frequency of April births in the MS population.

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