A Barrier to Understanding Teratogenicity: The Critical Periods of Sensitivity for Most Structural Birth Defects Precede the Established Hemochorial Placenta

This developmental chronology challenges the seemingly pervasive notion that placental transfer capacity plays a major role in mediating teratogenicity and highlights the need to further investigate the barrier capacity of the structures that surround and protect the developing embryo (e.g., trophoblast, yolk sac) prior to formation of the definitive placenta, and when the embryo is most sensitive to teratogenic insult.