Evidence that Microcephaly Risk is Highest among Pregnant Women Infected Early in Pregnancy

Colombia has experienced a 4-fold increase overall in microcephaly following the Zika virus outbreak in 2016, according to a study published today in the CDC’s Morbidity and Mortality Weekly Report. This preliminary report on Zika virus disease and microcephaly in Colombia demonstrates that an increase in microcephaly is not specific to Brazil. This finding confirms that countries with Zika virus outbreaks are likely to experience large increases in microcephaly and other Zika-related birth defects. The report also suggests that Zika virus infection in the first months of pregnancy poses the greatest risk of microcephaly.

From January 31 through mid-November 2016, there were 476 cases of microcephaly reported in Colombia; this represents more than a 4-fold increase from the same period in 2015, with a 9-fold increase in July 2016 (the peak month) compared with July 2015.  Of the 476 cases with microcephaly, 432 were live-born infants and 44 were pregnancy losses. 

According to the report, the peak in cases of microcephaly in Colombia came about 6 months after the period in which the highest number of new Zika infections was reported, which suggests that the highest risk period for Zika-associated microcephaly is likely to be in the first half of pregnancy, particularly the first trimester and early in the second. CDC recommends that women take steps to protect themselves during the entire pregnancy.

In general, Colombia reported approximately 105,000 cases of Zika virus disease, including nearly 20,000 cases in pregnant women, from August 9, 2015, through November 26, 2016. Zika virus infection during pregnancy can cause microcephaly (a smaller than expected head size) and other brain abnormalities in babies.

CDC and Colombia’s Instituto Nacional de Salud (INS) continue their collaboration to investigate microcephaly and other birth defects. This collaboration will provide critical information about Zika virus infection during pregnancy, including the range of health effects and the effect of other factors on pregnancy outcomes.

###
U.S. DEPARTMENT OF HEALTH AND HUMAN SERVICES