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Objectives Lead is one of the oldest known toxic metals. For decades, its effects on child development has been remained a topic of concern with an increased interest in ‘what prenatal blood lead levels should be considered toxic’. Many resent studies have shown the impacts of increased blood lead on different aspects of infants’ development at ‘acceptable’ levels (≤100μg/L). Methods To investigate the effects of prenatal lead exposure on children mental development, we have conducted a longitudinal study. Pregnant women (n = 364) who referred to hospitals for prenatal care at the first trimester of pregnancy were asked to participate in the survey. Maternal whole blood (MWB) samples, one for each pregnancy trimesters (3 times), and the umbilical cord blood samples, at the time of delivery, were collected and subjected to ICP-MS analysis for measurement of lead concentrations. We invited the mothers and their children to the research hospitals when the children were between 20 and 36 months of age and assessed mental development using Early Child Development Inventory (ECDI). The inventory included 60 items, which cover seven different development areas. Results MWB lead followed a U-shaped pattern over the course of pregnancy with lowest level during the second trimester. The ECDI score was inversely related to the first trimester blood lead concentrations (r = -0.15, p<0.05). The logistic regression analysis demonstrated significant relationships between increasing the first trimester lead concentrations (loge) with low score of ECDI, adjusting for multiple covariates (Unit risk: 5.7, 95% CI: 1.1 - 30.7, p <0.001). Conclusions Increased prenatal lead concentrations, even at “acceptable” level, adversely affects ECDI scores. Therefore, a reappraisal of lead exposure standards for female workers is a critical public health concern.

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