María D. Esteban-Vasallo, 1 Nuria Aragonés, 2, 3 Marina Pollan, 2, 3 Gonzalo López-Abente, 2, 3 and Beatriz Perez-Gomez2, 3
Background: Placental tissue may furnish information on the exposure of both mother and fetus. Mercury (Hg), cadmium (Cd), and lead (Pb) are toxicants of interest in pregnancy because they are associated with alterations in child development.
Objectives: The aim of this study was to summarize the available information regarding total Hg, Cd, and Pb levels in human placenta and possible related factors.
Methods: We performed a systematic search of PubMed/MEDLINE, EMBASE, Lilacs, OSH, and Web of Science for original papers on total Hg, Cd, or Pb levels in human placenta that were published in English or Spanish (1976–2011). Data on study design, population characteristics, collection and analysis of placenta specimens, and main results were extracted using a standardized form.
Results: We found a total of 79 papers (73 different studies). Hg, Cd, and Pb levels were reported in 24, 46, and 46 studies, respectively. Most studies included small convenience samples of healthy pregnant women. Studies were heterogeneous regarding populations selected, processing of specimens, and presentation of results. Hg concentrations > 50 ng/g were found in China (Shanghai), Japan, and the Faroe Islands. Cd levels ranged from 1.2 ng/g to 53 ng/g and were highest in the United States, Japan, and Eastern Europe. Pb showed the greatest variability, with levels ranging from 1.18 ng/g in China (Shanghai) to 500 ng/g in a polluted area of Poland.
Conclusion: The use of the placenta as a biomarker to assess heavy metals exposure is not properly developed because of heterogeneity among the studies. International standardized protocols are needed to enhance comparability and increase the usefulness of this promising tissue in biomonitoring studies.
During gestation, potentially harmful pollutants circulating in the blood of pregnant women can reach the fetus, although possible mother-to-child transmission of these toxic substances is modulated by the placenta. Some authors (Iyengar and Rapp 2001a; Subramanian and Iyengar 1997) have proposed the use of placental tissue as a noninvasive exposure biomarker for different organic and inorganic pollutants. This tissue, usually discarded after birth, is easy to obtain and may furnish information on the exposure of both mother and fetus.
Among the contaminants eligible for being studied in this tissue, some heavy metals are of special interest when it comes to gestation. Exposure to either mercury (Hg) or lead (Pb) has well known deleterious effects on the intellectual development of children (Jakubowski 2006), and embryotoxic and fetotoxic effects associated with exposure to cadmium (Cd) have also been described (Hazelhoff et al. 1987).
We reviewed published research on total Hg, Cd, and Pb levels in human placenta, both in pregnant women from the general population and in those with specific exposures, as well as the factors related to such levels, in order to summarize available information for researchers considering the use of placenta to biomonitor exposure to these metals.