Maternal and fetal exposure to lead, mercury, cadmium and manganese: the MIREC Study (Lay summary)

Arbuckle TE, Liang CL, Morisset AS, Fisher M, Weiler H, Mihai Cirtiu C, Legrand M, Davis K, Ettinger AS, Fraser WD, the MIREC Study Group. Chemosphere. 2016 Nov;163:270-82. doi: 10.1016/j.chemosphere.2016.08.023. 

Elevated levels of some metals can be harmful to health.  Lead (Pb) can be found in some food, drinking water and manufactured products. Sources of cadmium (Cd) include cigarettes and some foods. Mercury (Hg) is found in certain fish and silver dental fillings. Sources of manganese (Mn) include food and nutritional supplements, as well as air, water and soil.  Mn is an essential nutrient for human health, especially in pregnancy. As limited data are available on the levels of metals that Canadian women are exposed to during pregnancy, this study was designed to measure maternal and fetal exposure to Pb, Hg, Cd, and Mn.

Two biospecimens that can be used to estimate fetal exposure to chemicals are infant meconium and umbilical cord blood.  Meconium is the first stools that the newborn passes within the first few days of life and is composed of materials that the infant ingested while in the womb.  The umbilical cord vein connects the fetus to the placenta and carries blood to the fetus. The placenta is a temporary organ that connects the mother and fetus and transfers oxygen and nutrients from the mother to the fetus.

Nearly 2,000 women participated in the MIREC Study between 2008 and 2011. Metals were measured in 1st and 3rd trimester maternal blood, in umbilical cord blood at delivery and in infant meconium. Characteristics of participants (maternal age, education, annual household income, pre-pregnancy body mass index (BMI), parity, country of birth, and smoking status) were collected by questionnaire.  In addition, the women’s intake of calcium (Ca), iron and vitamin D was estimated based on the food they reported eating and the nutritional supplements they took during the pregnancy.

Most women had very low levels of Pb, Cd, Mn and total Hg in their blood. As commonly occurs in pregnancy, Mn levels generally increased over the course of the pregnancy and were highest in cord blood. Levels of Pb and Hg tended to be higher in cord blood than in mothers’ blood. Cd was rarely detected in cord blood or infant meconium, suggesting that it is not easily transported across the placenta to the fetus.  However, results suggest that Pb and Hg can relatively easily move across the placenta to the fetus.

Women who were less than 25 years of age, had less education (high school or less), were current smokers or with lower household income (<$50,000) had higher blood Cd levels. For all metals, levels in maternal blood were higher for women born outside of Canada.

Increasing calcium intake was associated with lower maternal Cd, Pb, Mn, and Hg, and cord blood Pb. Higher intake of vitamin D was associated with lower maternal Cd, Pb, and Mn, and cord blood Pb.

These results suggest that most women in the study were exposed to low levels of metals in pregnancy and that certain women, such as immigrants and smokers tend to have slightly higher levels of metals such as lead and cadmium in their blood.  The results also showed that some metals (e.g., lead, mercury) can be transferred from mother to fetus more easily than other metals (such as cadmium).  In addition, even at current low metal levels in blood, higher intake of calcium and vitamin D during pregnancy may be associated with lower maternal blood lead and cadmium concentrations and lower lead in cord blood.