Dams are susceptible to PPV-induced reproductive failure if infected anytime during about the first half of gestation. This interval of maternal susceptibility is indicated by the collective results of several experimental studies, by in-depth epidemiological investigations, and by estimates of the time of death of fetuses collected during epidemiological surveys. Consequences of maternal infection during this interval are embryonic and fetal death followed by resorption and mummification respectively. Transplacental infection also follows maternal exposure after midgestation, but fetuses usually survive without obvious clinical effects in utero. The likely reason is that transplacental infection often requires 10–14 days or longer, and by 70 days of gestation most fetuses are able to develop a protective immunologic response to the virus. In general, fetuses experimentally infected by transuterine inoculation of the virus have died when infected before day 70 of gestation, but they have survived and produced antibody when infected later in gestation. A strain of PPV of slightly greater virulence also has been reported. The usual consequences of infection at different stages of gestation are summarized in Table 1.