Abstract
The mechanism by which the Zika virus can cause fetal microcephaly is not known. Reports indicate that Zika is able to evade the normal immunoprotective responses of the placenta. Microcephaly has genetic causes, some associated with maternal exposures including radiation, tobacco smoke, alcohol, and viruses. Two hypotheses regarding the role of the placenta are possible: one is that the placenta directly conveys the Zika virus to the early embryo or fetus. Alternatively, the placenta itself might be mounting a response to the exposure; this response might be contributing to or causing the brain defect. This distinction is crucial to the diagnosis of fetuses at risk and the design of therapeutic strategies to prevent Zika-induced teratogenesis.