Estimates of Birth Defects Following Zika Virus Infection

On December 15, 2016, the CDC published a report regarding birth defects potentially related to Zika virus infection during pregnancy.  Based on a review of preliminary data from the US Zika Pregnancy Registry (USZPR), CDC scientists estimate that six percent of completed pregnancies following Zika virus infection were affected by one or more birth defects potentially related to Zika virus infection during pregnancy. The report combined data from the continental United States and Hawaii that were collected through a collaboration between the CDC and state and local health departments designed to monitor pregnancies with laboratory evidence of Zika virus infection.


As of September 22, 2016, 442 women with possible Zika virus infection in the USZPR had completed their pregnancies. Twenty-six of the completed pregnancies, or 6 percent, were reported to have one or more of the birth defects potentially related to Zika virus infection during pregnancy. Among women infected with Zika in the first trimester of pregnancy, 11 percent were reported to have fetuses or infants with birth defects, which is consistent with previous modeling estimates. The proportion of pregnancies with birth defects was similar for pregnant women who did or who did not experience symptoms, about 6 percent in each group. The 18 infants with a finding of microcephaly represent 4 percent (18/442) of the completed pregnancies; this prevalence is substantially higher than the background prevalence of microcephaly in the United States of about 7 per 10,000 live births, or about 0.07 percent of live births.


The 26 birth defects occurred among fetuses/infants of pregnant women who were exposed to Zika virus during their pregnancies in the following locations with active Zika virus transmission: Barbados, Belize, Brazil, Colombia, Dominican Republic, El Salvador, Guatemala, Haiti, Honduras, Mexico, Republic of Marshall Islands, and Venezuela.


“This is an important study. It shows that the rate of microcephaly and other fetal malformations related to Zika is similar among babies born in the United States – whose mothers were infected during travel to a dozen countries with active Zika transmission – to the estimated rate in Brazil,” said CDC Director Tom Frieden, M.D. M.P.H.. “Zika poses a real risk throughout pregnancy, but especially in the first trimester; it’s critical that pregnant women not travel to areas where Zika is spreading.”


These estimates should be considered in the context of a recent report of infants in Brazil with congenital Zika virus infection, meaning the Zika virus was transmitted from an infected mother to the unborn fetus or newborn infant some time during the pregnancy up through the time of delivery, who appeared healthy at birth, but later experienced slowed head growth and microcephaly. Given these recent findings from Brazil, it is possible that a greater proportion of the infants in this report could be affected by a Zika-related birth defect within the first year of life.


Prevention of Zika virus infection during pregnancy is critically important given the severity of its associated birth defects and the preliminary estimates outlined in this report. This report highlights the importance of CDC guidance to test all pregnant women with possible exposure to Zika virus regardless of whether they had symptoms of Zika, and to test infants born to mothers with possible Zika virus infection.


The CDC continues to recommend that pregnant women not travel to areas with Zika. If a pregnant woman travels to or lives in an area with active Zika virus transmission, she should talk with her healthcare provider and strictly follow steps to prevent mosquito bites and sexual transmission of Zika virus.


The CDC continues to encourage women considering pregnancy and their partners in areas with active Zika transmission to talk to their healthcare providers about pregnancy planning so that they know the risks and the ways to reduce them.


See the CDC Announcement


See also the CDC Report on Congenital Zika Virus Infection without Microcephaly at Birth in Brazil


See also the CDC’s Clinical Guidance for Healthcare Providers Caring for Pregnant Women


See also the CDC’s Clinical Guidance for Healthcare Providers Caring for Infants & Children


See also the CDC’s Information on Zika and Pregnancy


See also the CDC’s Information on Zika and Women Trying to Become Pregnant


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