Recently, there has been a significant amount of press (and fear) regarding the effects of the Zika virus on pregnant women and the risk of birth defects, most notably microcephaly. Since our understanding of the biology and effects of Zika virus are evolving, there are many questions that remain unanswered. If a women travels to an endemic area, what is the risk she will contract the virus? If she contracts the virus, what is the risk her fetus will be infected, and what is the risk of microcephaly or other fetal abnormalities? Is the Zika virus coming to the United States, and if so, when? Unfortunately, it will likely take more time to properly answer these questions.
In this week’s New England Journal of Medicine, Dr. Michael A. Johansson and colleagues from the Centers for Disease Control (CDC) published a perspective piece estimating the percent risk of microcephaly from Zika exposure. This was not a research study; meaning, they did not use specific patient data to come to their conclusions. Rather, they used the number of reported cases from Brazil and different infection-rate scenarios to estimate the risk for an individual woman infected with Zika virus per trimester. They concluded that for a pregnant woman infected in the first trimester, the risk of microcephaly would range from approximately 0.88% – 13.2%, based on different assumptions used in their calculation. The lower number is similar to the approximate 1% estimated risk that was seen in a prior 2007 outbreak in French Polynesia. For second and third trimester exposure, there was a negligible association between Zika infection and microcephaly, also similar to what was found for the French Polynesia outbreak.
This report provides the beginning for a true estimation of risk, but based on their calculations, it appears that first trimester infection has a low overall risk of microcephaly (probably less than 10% and maybe as low as 1%), and second/third trimester infection may not have a significant risk at all. However, there are other potential adverse outcomes from Zika infection, and more research needs to be done to elucidate them, as well as the risk with exposure per trimester. It is also important to realize that these data report the risk of microcephaly with actual Zika infection, and NOT simply travel to a Zika endemic area.
Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!