Twenty-Fold Increase in U.S. Birth Defects After Zika Infection Introduced

A CDC report published on March 3, 2017, found that the proportion of Zika-affected pregnancies with birth defects was approximately 20-fold higher compared with the proportion of pregnancies seen in 2013-2014, which is before Zika was introduced into the Americas. The types of birth defects—including brain abnormalities and/or microcephaly, neural tube defects and other early brain malformations, eye defects, and other central nervous system (CNS) problems—were seen in about three of every 1,000 births in 2013-2014. In 2016, the proportion of infants with these same types of birth defects born to women with Zika virus infection during pregnancy was about 6% or nearly 60 of every 1,000 completed pregnancies with Zika infections.


Zika virus infection causes serious brain abnormalities; however, the birth defects observed are not unique to congenital Zika virus infection, and the full range of effects of congenital Zika infection is not known. The researchers retrospectively assessed the prevalence of birth defects that met the surveillance case definition for birth defects potentially related to Zika virus infection before the Zika virus was introduced into the United States. Data on birth defects in the pre-Zika years serve as benchmarks to help scientists quantify birth defects potentially related to Zika virus infection. The higher proportion of these defects among pregnancies with laboratory evidence of possible Zika virus infection supports the relationship between congenital Zika virus infection and birth defects. This type of population-based birth defects surveillance allows scientists analyzing data related to a newly introduced teratogen, an agent or factor that causes birth defects, to take into account birth defects that already occur in the population and, presumably, would have occurred without the newly introduced teratogen.


The researchers analyzed 2013-2014 data from three birth defects surveillance programs in the United States—in Massachusetts, North Carolina, and Georgia—to provide the baseline frequency for Zika-related birth defects. To assess the effect of Zika virus infection during pregnancy, the scientists compared that 2013-2014 baseline number with previously published numbers among pregnancies with Zika virus infection from the U.S. Zika Pregnancy Registry (USZPR) from 2016.


They identified 747 infants and fetuses with one or more of these defects from programs in Massachusetts, North Carolina, and Georgia, from 2013-2014. Brain abnormalities and/or microcephaly were the most frequent conditions reported. Data from the USZPR identified 26 infants and fetuses with these same birth defects among the 442 completed pregnancies of women with possible Zika infection from January through September 2016.


These findings demonstrate the critical contribution of population-based birth defects surveillance to understanding the impact of Zika virus infection during pregnancy. In 2016, the CDC provided funding for 45 local, state, and territorial health departments to conduct rapid population-based surveillance for defects potentially related to Zika virus infection, which will provide essential data to monitor the impact of Zika virus infection in the United States. On March 3, 2017, five additional jurisdictions—Arkansas, Nebraska, Nevada, the Federated States of Micronesia, and the Republic of the Marshall Islands— were awarded funding, bringing the total to 50 jurisdictions and more than $27 million in support of birth defects surveillance for fiscal years 2016 and 2017.


See the CDC Announcement


See the CDC Report


See the CDC’s Zika site


See also Medical Law Perspectives, May 2016 Report: Vectors of Risk: Zika, West Nile, and Similar Tick and Mosquito Disease Litigation