Tofail et al. have varied and the effect size was generally small. It may be that duration of exposure is important. Previous studies in Bangladesh found a smaller effect of arsenic exposure in 6-year-olds (Wasserman et al. 2007) than in 10-year-olds (Wasserman et al. 2004). In the present study, arsenic concentration in urine was related to a wide range of socioeconomic variables. The association between arsenic exposure and circumstances of poverty is a common finding (Rahman et al. 2006), and the effect of poverty on child development is well documented and generally increases with age (Grantham-McGregor et al. 2007). It is difficult to measure all factors associated with poverty that affect child development. For example, few studies of arsenic exposure in school-age children have measured stimu- lation in the home, which is an important determinant of child development. However, the previous Bangladeshi study of 6-year-olds attempted to take stimulation into account and still found a small deficit in cognitive function associated with arsenic exposure (Wasserman et al. 2007). Arsenic exerts its toxic effects at least partly via oxidative effects (Chattopadhyay et al. 2002). In the present study, all mothers received folate in pregnancy, and a possible explanation for the lack of a detectable effect is that there was some protection due to folate, which has antioxidant properties (Mukherjee et al. 2006). Another protective mechanism is the induced metabolism of arsenic in preg- nancy (Vahter 2007). Inorganic arsenic, the main form in the drinking water, is methylated via one-carbon metabolism, producing methy- larsonic acid (MMA) and dimethylarsinic acid (DMA), which are the main metabolites excreted in urine (Vahter 2002). Although a high percentage of MMA in urine is considered a risk factor for a wide range of toxic effects of arsenic (e.g., Li et al. 2008; Tseng 2007), a high percentage of DMA is associated with increased rate of excretion and fewer health risks. We found that the mothers of the studied infants had remarkably efficient methylation of arsenic already in early pregnancy (Li et al. 2008), and it is likely that further improvement occurred with advancing gestation (Concha et al. 1998; Hopenhayn et al. 2003). The present findings are likely to reflect mainly intrauterine exposure, because arse- nic readily crosses the placenta and breast milk excretes very little arsenic (Concha et al. 1998). In a subsample in this cohort, the median concentration of arsenic in breast milk was 1 µg/L (Fangstrom et al. 2008). In this cohort, exclusive breast-feeding lasted 4 months on average, and essentially all children were partly or predominantly breast-fed at the time of the testing at 7 months (Saha et al. 2008). However, around half the children were taking semisolids or solids at 6 months of 292 age, mostly begun after 5 months. Therefore, a limitation to the study is that the children would have had some small exposure to other sources of arsenic. Conclusion In conclusion, we failed to detect significant effects of prenatal arsenic exposure on infants’ problem-solving abilities, motor development, and behavior. It is possible that other cog- nitive functions are affected and that effects may appear at a later age. We intend to follow these children to determine if deficits occur in the future. There remains an urgency to reduce arsenic exposure in pregnancy owing to the increase in prenatal and infant mortal- ity previously reported in this area (Rahman et al. 2007). 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