Second, quantifying the relative and absolute risks among infants who are born to women who were infected at different times during pregnancy. Third, identifying factors that modify the risk of an adverse pregnancy or birth outcome (e.g., coinfection with another virus, preexisting immune response to another flavivirus, genetic background of the mother or fetus, and severity of infection). Addressing these issues will improve our efforts to minimize the burden of the effects of Zika virus infection during pregnancy.