The monitoring of Vitamin D deficiency during pregnancy is very important and has become a general practice in many countries. Total 25OH Vitamin D is typically measured two times during pregnancy and supplements are prescribed according to the observed levels.
Fetal needs for vitamin D increase during the latter half of pregnancy, when bone growth and ossification are most prominent. Vitamin D is transported to the fetus by passive transfer, and the fetus is entirely dependent on maternal stores. Therefore, maternal status is a direct reflection of fetal nutritional status.
Due to a fast and large increase in the concentration of the Vitamin D Binding Protein during pregnancy the measurement of total 25OH Vitamin D is no longer a good representation of the concentration of the free 25OH Vitamin D.
The total 25OH Vitamin D test overestimates the free fraction, with the risk of missing Vitamin D deficiency or insufficiency.
Recent research has shown that the measurement of free 25OH Vitamin D during pregnancy is a much better marker of Vitamin D deficiency than the actual practice and should be implemented in the clinical laboratories.
FREE 25(OH) VITAMIN D reflects better the 25OH Vitamin D status IN NORMAL PREGNANCY
On March 19th, 2018, Hocher et al. published a paper entitled “Reference intervals for measured and calculated free 25-hydroxyvitamin D in normal pregnancy.”
https://www.ncbi.nlm.nih.gov/pubmed/29567112
This is the result of the largest study comparing free- and total 25(OH) Vitamin D during pregnancy. The following differences were found between both measurements: