Maternal exposure to arsenic and mercury and associated risk of adverse birth outcomes in small-scale gold mining communities in Northern Tanzania

•Our paper reports on adverse pregnancy outcomes of women living in artisanal gold mining communities in Tanzania.•We highlight the prospective longitudinal study design which obtained pregnancy outcomes on 961 women and the use of direct measurement of mercury and arsenic levels during the pregnancy.•Just over 50% of women living in these gold mining communities had an adverse pregnancy outcome. Exposure to arsenic and mercury in artisanal and small-scale gold mining (ASGM) communities is an issue that predominantly affects low and middle-income countries. Large epidemiology studies in these communities are rare, and the impact of such exposures on reproductive outcomes are not well understood. To examine associations between prenatal maternal arsenic and mercury exposure and birth outcomes in both ASGM and non-ASGM communities in Northern Tanzania. This longitudinal prospective study included 961 women (ASGM = 788, non-ASGM = 173) of the original cohort of 1056 who were followed until a pregnancy outcome was registered. Maternal spot urine samples and dried blood spots were used to measure total arsenic (T-As) and total mercury (T-Hg) in the second trimester of pregnancy. Data on adverse birth outcomes were collected in 5 categories: spontaneous abortion, stillbirth, preterm birth, low birth weight, and visible congenital anomalies. Mann-Whitney U-tests were used to test for differences between median T-As and T-Hg by area of residence. Logistic regression models were used to estimate the odds of stillbirth and visible congenital anomalies given maternal T-As and T-Hg levels. Modified Poisson regressions were used to estimate relative risk ratios between maternal T-As and T-Hg levels and composite adverse birth outcome, spontaneous abortion, low birth weight, and preterm birth. Statistically significant differences were found in median T-As (9.6 vs. 6.3 µg/L, Mann-Whitney U-tests, Z = −3.50, p