Both pregnancy and high vitamin D concentration seem to generate a protective environment against multiple sclerosis (MS) relapses. Longitudinal case–control analysis of vitamin D concentrations during pregnancy and lactation of MS mothers is lacking.

To examine serum 25-hydroxyvitamin-D₃ levels of MS patients during and after pregnancy and compare these to the levels measured in healthy controls.

Fifteen relapsing–remitting MS mothers underwent repeated testing for 25-hydroxyvitamin-D₃ at 10–12, 26–28 and 35–37 gestational weeks and 1, 3 and 6 months post-partum. An identical series of samples was collected from six control mothers.

The prevalence of vitamin D deficiency (<50 nmol/l) during pregnancy was high (73%) among MS patients. Vitamin D levels were significantly higher during pregnancy when compared to early post-partum values among MS patients. At the end of the follow-up period, the vitamin D levels returned to levels observed in early pregnancy. In healthy controls, the alterations during and after pregnancy were similar in nature, but the vitamin D concentrations were higher at all time points when compared to MS patients (P = 0.037).

Vitamin D deficiency during the pregnancy and lactation seems to be common in mothers with MS and needs to be treated adequately.