CDC Releases Estimates of Birth Defects Among Fetuses & Infants of US Women with Evidence of Possible Zika Virus Infection During Pregnancy

Although it has been confirmed that congenital infection with the Zika virus can cause microcephaly, among other neurological complications, the exact risk of developing such complications due to the infection is still unknown.

In a recent study, Margaret Honein, PhD, and her colleagues from the Centers for Disease Control and Prevention (CDC), estimated the percentage of congenitally-infected fetuses or infants that developed Zika-related complications, based on which trimester the mother was infected with the Zika virus and what symptoms she experienced. Six percent of fetuses or infants congenitally infected with the virus were reported to have developed birth defects.

The study, published in the Journal of the American Medical Association (JAMA), used data reported to the US Zika Pregnancy Registry, a data collection registry launched by the CDC in early May 2016 that contains information on all reported cases of Zika infection in pregnant women from the continental United States and Hawaii.

According to the report, between January 15 and September 22, a total of 442 women in the United States were reported to have contracted Zika virus infections while pregnant. These pregnancies resulted in 47 “pregnancy losses” (not specified to be either miscarriages or abortions), and 395 live births. Among the pregnancy losses, 5 fetuses expressed Zika-related complications, while 21 of the live births showed birth defects. Four of the fetuses/infants who were reported to have Zika-related complications, were diagnosed with microcephaly but did not undergo neural imaging to determine other neurological symptoms; in addition, 14 were reported to have microcephaly and brain abnormalities, while four were diagnosed with brain abnormalities but not with microcephaly. A press release on the study confirms, “Birth defects were reported for 16 of 271 (6 percent) pregnant asymptomatic women and 10 of 167 (6 percent) symptomatic pregnant women.”

Nine of the women in the registry who experienced symptoms consistent with Zika infection during the first trimester reported having infants or fetuses with birth defects; however, there were no reported birth defects observed in infants born to mothers who were infected with the virus in the second or third trimesters. This further supports previous claims that a Zika virus infection during the first trimester causes the most harm to the developing fetus.

In comparison with microcephaly rates from a 2009-2013 US registry, which reported 7 cases for every 10,000 live births, these findings showed a “prevalence that is substantially higher than the background prevalence of microcephaly,” notes the JAMA press release.

Although none of these women examined in the study were reported to have been infected with Zika within the United States, all were infected in regions with confirmed active Zika virus transmission; these areas are included on the CDC’s list of regions to avoid travel to while pregnant.

Tom Frieden, MD, MPH, director of the CDC stated, “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. 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.”