Objective: Prematurity is an important etiologic factor for perinatal mortality and morbidity. In our country, preterm births and complications of prematurity account for 26% of perinatal mortality. There are many maternal, fetal or placental, genetic and environmental etiologic factors that cause prematurity. There have been no studies to show an association between preterm delivery of unknown etiology and exposure to heavy metals and trace elements at toxic levels in meconium. The purpose of this study is to measure the levels of heavy metals (lead, cadmium) and trace elements (zinc, iron, copper) in meconium samples and to understand their associations with preterm delivery of unknown etiology. Material and Methods: The levels of heavy metals and trace elements in the meconiums of 810 term or preterm infants with known and unknown etiology for being preterm were measured with a flame atomic absorption spectrophotometer. Results: Lead and cadmium were detected in all meconium samples. Heavy metal and trace element levels in meconium were significantly higher in preterms of unknown and known etiology for being preterm compared to term infants (for all p<0.0001). Lead levels in meconium were significantly higher in preterm of unknown etiology compared to preterm of known etiology in posthoc analysis with Bonferroni corrected Mann Whitney U test. Conclusion: These results may suggest that lead levels in meconium samples are higher in preterm newborns especially with unknown etiology compared to term newborns. Therefore, we suggest that preterm delivery of unknown etiology may be decreased by decreasing air pollution.