The effects of season and stage of pregnancy on plasma 25-hydroxy-vitamin D concentrations in pregnant women.

Plasma 25-OHD levels were measured in the same 26 Caucasian women before pregnancy and during the first, second and third trimesters of their pregnancy. This was done to assess whether pregnancy alters vitamin D status in healthy women. During most times of the year there was no association between the 25-OHD levels and the stage of pregnancy, but between January and March there was a progressive fall in 25-OHD levels with each trimester. When 25-OHD levels were related to hours of possible sunlight exposure a negative association between 25-OHD levels and stage of pregnancy was noted only in subjects with the lowest possible exposure to sunlight. These observations suggest that pregnancy has an effect on vitamin D metabolism, but that in healthy Caucasian women these effects only become manifest where there is a low level of exposure to sunlight.