In a study of 1,400 pregnant women, those who had high hemoglobin levels early on faced nearly double the risk of giving birth to a stillborn child – a fetus born dead more than five months into pregnancy.
High levels of hemoglobin – an oxygen-carrying protein that gives red blood cells pigment can be caused by cigarette smoking, which also has been linked to stillbirths. But this study found no clear association between smoking and
hemoglobin and lacked data on what caused participants’ high levels.
The findings, from Dr. Olof Stephansson and colleagues at Karolinska Institute in Stockholm, appear in Wednesday’s Journal of the American Medical Association.
Problems with the placenta and birth defects are among causes of the estimated 250,000 U.S. stillbirths each year, but more than 80,000 are due to unknown causes. The findings suggest that closely monitoring pregnant women with high hemoglobin levels could help prevent some of those deaths, said Dr. Nancy Green, associate medical director of the March of Dimes, a charitable group that supports research on birth defects and child mortality.
The study "underscores the importance of prenatal visits," said Green, who is also an assistant professor of pediatrics and cell biology at Albert Einstein College of Medicine in New York.
Hemoglobin levels are measured in blood tests routinely given to pregnant women during prenatal visits. Average levels for early in pregnancy should be around 13.3 grams per deciliter, Green said. Levels linked with the increased risk – 14.6 grams per deciliter or higher – were found at visits during the 10th week of pregnancy. High levels found later in pregnancy did not seem to increase the risk of stillbirth.
Blood containing too much hemoglobin tends to be sticky and less able to flow through tiny blood vessels. That could disrupt blood flow to the placenta or developing fetus, although more research is needed to explain how high levels
might cause stillbirths, Green said.
Extremely high hemoglobin levels could be due to rare heart defects in the mother. But those deemed high in the study also could be considered at the uppermost limit of normal and thus might not be recognized as potentially problematic, she said.
Anemia – abnormally low levels – is a potentially serious and far more common pregnancy complication, and women often are advised to take iron supplements to avoid it. Green said the amount of iron normally prescribed for pregnant women
would not cause abnormal hemoglobin increases. He cautioned that the findings should not be interpreted to mean pregnant women should stop taking iron.