While this is the first study evaluating the effects of dental filling materials on low birth weight, some important questions remain unanswered. This study was not designed to evaluate the effect of mercury on chromosomal alterations, neurobehavioral characteristics, or other possible adverse pregnancy outcomes such as spontaneous abortions. In addition, this study was not designed to evaluate the effects of resin-based filling materials on pregnancy outcomes. The safety profile of resin-based filling materials is considerably more difficult to establish because of significant product variability and analytical complexity. Compositebased filling materials are of concern because of the presence of estrogen-like molecules, which are an environmental pollutant and which have been associated with chromosomal abnormalities in mice, fetal loss, and an increased birth weight soon after birth (4), as well as with underdevelopment of the brain, kidney, and testes in mice (26).