The team said other studies had suggested exposure to the sun or seasonal variations in a mother's vitamin D levels during pregnancy may have an impact on brain development.
Children born in November & December in northern countries had a lower risk of developing MS as adults, May was a bad month. This suggests the disease may have a quite early onset, but only manifest later in life.
I like the sunlight theory myself. Twenty years ago I speculated about this as a medical student. It's the obvious explanation for the correlation between latitude and disease incidence; the theory must have occurred to thousands of students over the past forty years. I thought then that it might be related to cutanous immune cells. There is a lot of curious immunology that appears to go on in the skin and it makes sense that it could be affected by radiation exposure and by the adaptations to radiation exposure.