The role of the placenta in the relationship of maternal and fetal exposure to endocrine disrupting chemicals (EDC), and the long-term effects on the health of the child is unknown. Yet knowledge of the role of the placenta can increase temporal and spatial precision in estimating health risks to the fetus. A class of EDCs called phthalates, are detected in all pregnant women in the U.S. Potential roles for the placenta in phthalate toxicity include transfer of phthalate metabolites to fetal circulation, phthalate-induced damage to placental development and function (nutrient transfer, invasion of the myometrium, oxygen regulation, immune function, parturition signaling), or hormonal mediation of fetal programming. Based on findings from a pilot study of placental gene expression and maternal phthalate exposure, we hypothesized that phthalates disrupt trophoblast differentiation (GCM1, PPARG, AHR, FATP, CGA). This would consequently alter placental function over the course of the pregnancy and possibly explain correlations between prenatal exposure and outcomes at birth. In the K99 phase, we conducted experimentation to test this hypothesis using two primary placental cell models of the undifferentiated and differentiated primary trophoblasts, dosing cells with the same concentrations of mono-n-butyl phthalate (MnBP) measured in prenatal urine. We found that a) the transcriptome response of the cells to MnBP differed by the sex of the fetus; b) MnBP at 150-200 nM increased the secretion of a placenta-specific hormone called human chorionic gonadotropin (hCG); c) and the effects of MnBP on the trophoblast overall were pro-differentiation and pro-invasiveness. Based on these insights, in the R00 phase we focused on the sex-specific molecular response of the placenta and the potential for placental mediation of fetal programming. In Aim 1, we experimentally confirmed the sex-specific effects of phthalates on trophoblast gene and protein expression, and specifically on hCG synthesis and secretion. In Aim 2, we translated our experimental findings back to an epidemiologic context in collaboration with a birth cohort study (TIDES) and by sampling maternal and placental tissue from first trimester pregnancy terminations (e3pad). In conclusion, we have confirmed the sex-specific effects of phthalates on the placental transcriptome, hCG transcription, translation and secretion in vitro and in human pregnancy. Secondly, we have demonstrated using observational data that the first trimester placenta may mediate the effects of phthalates on fetal sex differentiation by way of placental hormonal disruption. Thirdly, we have generated a bank of placental tissue samples from the TIDES cohort that can be used to further probe hypotheses regarding the fetal origins of endocrine disruption. ExpandCollapse Abstract