http://www.medpagetoday.com/MeetingCoverage/AAN/25977

HONOLULU -- Mothers' exposure to sunlight and ultraviolet radiation during pregnancy was associated with their children's future likelihood of developing multiple sclerosis, a researcher said here.

This is the second study to report a link between in-utero sun exposure and MS risk -- but the earlier study found an association for first trimester risk while Giulio Disanto, MD, of the University of Oxford, England, said that exposure during the second and third trimester was key.

Pregnant women in Great Britain with relatively high UV exposure during the second and third trimesters were significantly less likely to deliver children who were later diagnosed with multiple sclerosis, Disanto reported.

Similarly, low vitamin D levels in mothers-to-be late in pregnancy were associated with increased MS risk in their offspring, he explained at a late-breaking poster presentation here at the American Academy of Neurology's annual meeting.

Low levels of vitamin D during gestation -- either because of inadequate nutrition or exposure to the sun -- have been associated in several studies with increased risk of adult multiple sclerosis.

In particular, the findings by Disanto and colleagues echo those of an Australian study reported last year, except that study found that UV exposure during the first trimester was the key factor.

Disanto said it was unclear why the two studies did not agree on the exposure-risk relationship's timing, although their methodologies differed considerably.

The Australian study relied on birth month and climatological averages to estimate gestational UV exposure over a 26-year period.

Disanto and colleagues, on the other hand, obtained weather satellite data indicating monthly average UV exposure in Great Britain during the birth years of the study, 1979 to 1992.

These data were then correlated with gestational months for 11,282 MS patients diagnosed from 1997 to 2009 and more than 2 million controls in Scotland, plus 8,702 cases and about 20,000 controls in England and Wales who had died from 1971 to 1991.

Disanto acknowledged that the satellite data did not correspond to the actual birth years of the people in the study, but he said the averages have not changed much over time.

Average monthly serum 25-hydroxyvitamin D (25-OH-D) was obtained from two other previous studies, involving some 3,400 women in England and Wales and 354 in Scotland.

Confirming previous studies, individuals born in the spring -- when their second and third gestational trimesters were in the winter months -- had higher rates of later MS diagnosis, the researchers found.

These differed somewhat in Scotland versus England and Wales. In the more northerly area, the highest rates of MS were seen with April and May births (odds ratios 1.07 to 1.09) and the lowest with September birth (OR 0.92), all with P<0.05.

In England and Wales, the only significant spike in MS risk was in May births (OR 1.17, P<0.001) and the troughs occurred with November and December births (both OR 0.88, P=0.01).

These results corresponded to significant increases in MS risk with the lowest UV exposure during the fourth to seventh months of pregnancy in Scotland and the third to sixth months in England and Wales.

MS risk was also highest with low 25-OH-D levels occurring in the fifth to eighth gestational months in England and Wales and during the final months in Scotland.

Disanto told MedPage Today that the growing body of evidence linking low sun exposure and 25-OH-D to MS risk was enough to warrant vitamin D supplementation during pregnancy.

He suggested that up to 4,000 mg/day would be suitable -- 10 times the current USDA-recommended intake -- arguing that it was adequately safe and that it was important to get enough to ensure adequate fetal exposure.