There is a significant fall in the serum folic acid level during pregnancy, reaching its lowest level at term. This is most pronounced in twin pregnancies. A similar but less spectacular fall occurs in the vitamin B12 concentration.
In megaloblastic anaemia both folic acid and vitamin B12 levels are lower than in other pregnant women. The degree of megaloblastic change in the bone marrow, as measured by the type and number of megaloblasts, is reflected in the vitamin levels, cases with florid megaloblastosis showing the most marked depression of vitamin B12 and folic acid activity.
Although there is a significant difference in the mean folic acid levels between megaloblastic and normoblastic pregnant women, a considerable overlap exists between individual values in the two groups. When the labile folic-acid factor is determined separately the test becomes much more specific. In the present series, all cases of megaloblastic anaemia yielded labile-factor levels below 1·0 mμg. per ml., while a similar value was encountered in only one of 35 normal pregnancies.
In five women with megaloblastic anaemia the vitamin B12 concentration was less than 100 μμg. per ml. but rose to normal levels on folic acid therapy alone.
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