Is there a relation between stillbirth and low population vitamin D levels? -a binational follow-up study of vitamin D fortification

Background: Stillbirth has been associated with low plasma vitamin D. Both Sweden and Finland have been characterized by a high proportion of low plasma vitamin D levels (< 50nmol/L). We aimed to access the odds of stillbirth in relation to changes in National vitamin D fortification. Methods All pregnancies in Finland between 1994 and 2021 (n = 1,569,739) and Sweden (n = 2,800,730) with live or stillbirth registered in the Medical Birth Registries. The mean incidences before and after changes in the vitamin D food fortification programs in Finland (2003 and 2009) and Sweden (2018) were compared with cross-tabulation with 95% confidence intervals. Results In Finland, there was a reduction of stillbirth rate was reduced from ~ 4.1/1000 before 2003, to 3.4/1000 2004–2009 (odds ratio [OR] 0.87, 95% CI 0.81–0.93) and 2.8/1000 after 2010 (OR 0.84, 95% CI 0.78–0.91). In Sweden, the stillbirth rate decreased from 3.9/1000 2008–2017 to 3.2/1000 after 2018 (OR 0.83, 95%CI 0.78–0.89). The dose-dependent difference in Finland in a large sample with correct temporal associations, when the level decreased in Finland, it was steady in Sweden, and vice versa), indicate that the effect may be due to vitamin D. The findings should be interpreted as observational and may not be causal. Conclusion Each increment of vitamin D fortification was associated with a ≈ 15% fewer stillbirths on a national level. If true, and fortification reach all, it might be a milestone in preventing stillbirth and reduce health inequalities.