Several human studies have shown that low-level exposure to environmental contaminants, such as polychlorinated biphenyls (PCBs) and organochlorine pesticides, negatively influences birth outcomes. However, the effects of low-level exposure to polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and dioxin-like PCBs (DL-PCBs) on birth outcomes have not been clarified in human studies. A prospective cohort study was established to investigate the possible adverse effects of PCDDs/PCDFs and DL-PCBs on fetal growth and neurodevelopment. We recruited 514 pregnant women between July 2002 and October 2005 in Sapporo, Japan. We measured 29 congener levels of PCDDs/PCDFs and DL-PCBs in maternal blood. Using multiple liner regression analysis of the association between birth weight and the levels of PCDDs/PCDFs and DL-PCBs with full adjustments for potential confounders, a significant adverse effect was observed regarding total PCDDs toxic equivalents (TEQ) levels (adjusted beta=-231.5g, 95% CI: -417.4 to -45.6) and total PCDFs TEQ levels (adjusted beta=-258.8g, 95% CI: -445.7 to -71.8). Among male infants, significant adverse associations with birth weight were found for total PCDDs TEQ level, total PCDDs/PCDFs TEQ level, and total TEQ level. However, among female infants, these significant adverse associations were not found. With regard to individual congeners of PCDDs/PCDFs and DL-PCBs, we found significantly negative association with the levels of 2,3,4,7,8-PeCDF (adjusted beta=-24.5g, 95% CI: -387.4 to -61.5). Our findings suggest that prenatal low-level exposure to PCDDs and PCDFs, especially 2,3,4,7,8-PeCDF, may accumulate in the placenta and retard important placental functions, which result in lower birth weight.