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Background Thousands of pregnant women are exposed to arsenic (As), which has been shown to lead to a higher risk of maternal and infant morbidity. We hypothesized that As-induced modifications to the humoral immune system may be partly responsible, and examined the relationship between As and immunoglobulin G (IgG). Methods Pregnant women were recruited in As-contaminated rural areas in Bangladesh. Blood and urine samples, and questionnaire data were collected. We analyzed data from 202 pregnant women and a subset of 121 mother–infant pairs. Urinary As was measured on inductively coupled plasma–mass spectrometry and adjusted with specific gravity (U-AsSG). Maternal (IgGmat) and cord (IgGcord) serum total IgG were measured using immunoturbidity assay. Results The geometric mean U-AsSG (n = 202) was 69 μg/L (range, 3.1–1356 μg/L). Urinary-AsSG was significantly associated with IgGmat (n = 202; (r = 0.24; 95% confidence interval [CI]: 0.10–0.36; P < 0.001) and remained so after the inclusion of maternal-associated variables in a multiple-regression model (β = 1.26; 95%CI: 0.47–2.05; P < 0.01). U-AsSG, however, was not significantly associated with IgGcord (n = 121), while IgGmat and IgGcord were also not associated with each other. Conclusions Maternal As exposure was positively associated with IgGmat but not IgGcord. Elevated IgGmat may have implications as regards maternal morbidity and the placental transfer of specific IgGs. Further studies are required to better understand how As may affect maternal and child health by modifying the humoral immune system.