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.
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.