Background: Cadmium (Cd) is an embryotoxic and teratogenic metal in a variety of animal species, but data from humans are limited. Objectives: The aim of the present study was to assess the effects of maternal Cd exposure in pregnancy on size at birth. Methods: This prospective cohort study was nested in a population-based nutritional supplementation trial in pregnancy conducted in rural Bangladesh. We selected women recruited from February 2002 through January 2003 who had a singleton birth with measurements of size at birth and had donated a urine sample in early pregnancy for Cd analyses (n = 1,616). Urinary Cd was measured with inductively coupled plasma mass spectrometry and adjusted for specific gravity. Results: Multiple linear regression analyses adjusted for sex and other potential confounders showed that maternal urinary Cd (median, 0.63 μg/L) was significantly negatively associated with birth weight [unstandardized regression coefficient B = –31.0; 95% confidence interval (CI): –59, –2.8] and head circumference (B = –0.15; 95% CI: –0.27, –0.026). However, associations appeared to be limited to girls, with little evidence of effects in boys. A 1-μg/L increase in Cd in maternal urine was associated with a 0.26-cm (95% CI: –0.43, –0.088 cm) and 0.24-cm (95% CI: –0.44, –0.030 cm) decrease in girls’ head and chest circumferences, respectively, and a 45-g (95% CI: –82.5, 7.3 g) decrease in birth weight. Quantile regression analyses indicated that associations with maternal Cd were similar for girls of smaller (25th percentile) and larger (50th and 75th percentiles) sizes at birth. Conclusion: We found evidence of a sex difference in the association between maternal Cd exposure and birth size, which was apparent only in girls. Results add support for the need to reduce Cd pollution to improve public health.