The exact reasons are not known, but it is likely to be the result of several processes. The leading explanations are hemodilution[1] due to the increase in blood volume during pregnancy, decreased exposure to the sun, and the fetal demand for vitamin D, because it is an essential nutrient.
References
- ^Kaibara M, Marumoto Y, Kobayashi THemodilution and anemia in pregnancy and fetal developmentNihon Sanka Fujinka Gakkai Zasshi.(1984 Oct)