Vitamin B-12, an essential nutrient required for the production of red blood cells and normal function of the nervous system, assists in fetal growth and development during pregnancy. A lack of B-12 causes anemia and nerve damage in the brain and spinal cord. A deficiency during pregnancy increases the risk of miscarriage, and may have harmful effects on the fetus. The B-12 level in blood is low when the vitamin is deficient; however, it is a less accurate indicator of overall body stores during pregnancy.
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Low B12 Levels
Vitamin B12 levels fall progressively throughout pregnancy in most healthy women. The decline begins in the first trimester and levels may drop to 50 percent of pre-pregnancy values by the third trimester. Vitamin B-12 levels below the normal range are found in 35 percent to 43 percent of pregnant women at delivery, but do not indicate a total body deficit in the majority of cases.
The total fetal B-12 requirement of 50 mcg does not significantly deplete the normal maternal store of approximately 3,000 mcg. Several other observations also suggest low B-12 levels during pregnancy do not reflect a true tissue deficiency. Low levels return to normal pre-pregnancy values promptly after delivery without B-12 supplements. Pregnant women with low levels usually do not have anemia or other signs of a deficiency.
Methylmalonic acid and homocysteine are substances which build up in blood when B-12 is deficient. High levels are found in 98 percent of patients with depleted B-12 tissue stores. In contrast, levels are usually not elevated in pregnant women with low B-12 levels. In a 2007 study in the Journal of Nutrition, B-12 levels decreased throughout pregnancy, but methylmalonic acid levels remained within the normal range.
The normal physiologic changes during pregnancy contribute to the fall in B-12 levels. The volume of plasma, the liquid part of blood, increases by 40 to 50 percent, which lowers the vitamin concentration. Because the kidneys filter blood at a higher rate during pregnancy, more B-12 may be lost in the urine. B12 is actively transferred across the placenta to the developing fetus. Vitamin B-12 levels in umbilical cord and fetal blood are twice as high as in maternal blood, indicating preferential delivery to the fetus.
Vitamin B-12 levels vary by as much as 136 ng/liter when testing is repeated in a single individual. A low level in a pregnant woman does not necessarily indicate a deficiency. The drop in levels without other evidence of a deficiency suggests the normal reference range for non-pregnant individuals may not be accurate for assessing B-12 status during pregnancy. Other factors besides a deficiency can affect blood levels. For example, B-12 levels are low in more than 10 percent of people with folic acid deficiency.
A low B-12 level can uncommonly reflect a true deficiency during pregnancy. The risk is higher in strict vegetarians, women who have undergone weight loss surgery and those with intestinal diseases that prevent B-12 absorption. The diagnosis of B-12 deficiency is based on other laboratory tests in addition to a low blood level. Abnormally large red blood cells appear at an early stage. A persistent lack of B-12 causes anemia, a deficiency of red blood cells. Elevated methylmalonic acid and homocysteine levels strongly suggest a true deficiency. Normalization of these high values and correction of anemia after B-12 replacement confirms the diagnosis.
Vitamin B-12 levels in breast milk are similar to those in maternal blood. Because B-12 secretes into breast milk, the recommended dietary allowance for breastfeeding is slightly higher than during pregnancy. Inadequate B-12 intake during pregnancy or postpartum may lower the concentration in breast milk, even in women with sufficient total body stores. Low B-12 levels, 10 percent to 30 percent of normal, are found in breast milk from vegan or long term vegetarian mothers. Their infants, if exclusively breastfed, are at risk for B-12 deficiency. A vitamin supplement increases the B-12 level in breast milk if maternal intake is low.