Maternal and cord blood manganese (Mn) levels and birth weight: The MIREC birth cohort study (Lay summary)

Ashley-Martin J, Dodds L, Arbuckle TE, Ettinger AS, Shapiro GD, Fisher M, Monnier P, Morisset AS, Fraser WD, Bouchard MF. International Journal of Hygiene and Environmental Health. 2018 Jul;221(6):876-882. doi: 10.1016/j.ijheh.2018.05.015.

Low birth weight has been linked to a number of adverse health outcomes among infants and throughout childhood. As of 2013, nearly 7% of children born in Canada have low birth weight (defined as weighing less than 2500 grams, or about 5 pounds, 8 ounces). While there are many factors that are involved in regulating birth weight, fetal exposure to manganese in utero (i.e., during pregnancy) is one factor that is not well understood. Manganese is a naturally occurring metal that is necessary for ideal fetal development. This essential nutrient occurs widely in our food, so nearly all women have detectable levels of manganese in their blood. Other sources of exposure include air, water, and soil. Some studies have shown that both low and high levels of manganese during pregnancy may be associated with low birth weight. It is not known whether these effects occur at the levels that are typically experienced by Canadian women.

To answer this question, researchers from Dalhousie University led an analysis of data from the MIREC Study. The aim was to examine the relationship between high and low blood levels of manganese during pregnancy and birth weight. Maternal blood from the first and third trimesters of pregnancy in about 1,500 women, as well as umbilical cord blood, were analysed for manganese. Manganese levels tend to increase through pregnancy and be even higher in cord blood. Information on the birth weight and gestational age at delivery for singleton term infants was available from the questionnaires and medical charts of women in the study.

Women with low (<10th percentile, or <0.82 µg/dL) manganese levels in the third trimester tended to have infants with lower birth weight – about 83 grams lower on average. Importantly, this association was only statistically significant when concurrent exposure to heavy metals, including lead, arsenic, mercury, and cadmium, was included in statistical models. There may also be a link between high umbilical cord blood levels of manganese and lower birth weight. No statistically significant findings were observed between first trimester or umbilical cord blood manganese levels and birth weight.

In this group of Canadian women, fetal exposure to manganese – particularly occurring during the third trimester – may be one of the many factors that influence birth weight. Further research is needed to confirm these findings.