Relatives of Zika virus can damage developing fetuses in mice and were able to replicate in human maternal and fetal tissues, researchers report. Their findings suggest that other emerging viruses circulating in the Western Hemisphere might have the capacity to cause birth defects. Even though Zika virus was first identified more than 70 years ago, its ability to induce birth defects (including microcephaly) was not recognized until the massive 2015 South American epidemic, which sickened more than 1.5 million people. Speculating that other insect-transmitted pathogens belonging to the flavivirus family might have similar potential to cause harm, Derek Platt and colleagues tested the ability of West Nile virus (WNV), Powassan virus (POWV), chikungunya virus (CHIKV), and Mayaro virus to infect the placenta and fetus in pregnant mice with fully intact immune systems. Although all four viruses crossed the placenta and replicated inside the fetal brain, only WNV and POWV caused lethality to the developing fetuses. WNV (a mosquito-borne virus that is has been detected across the continental United States) caused neural progenitor cell death in developing fetuses, a phenomenon not observed for CHIKV. Similar to Zika virus, WNV and POWV (a tick-transmitted disease, most commonly reported in the Great Lakes region) replicated in samples of both maternal and fetal tissues obtained from healthy pregnant human donors in their second trimesters. Based on their findings, the authors speculate that flavivirus infections might cause pregnancy complications more frequently than currently appreciated.