Microcephaly is typically a very rare condition, causing about seven babies per 10,000 births to be born with undersized heads. This can lead to vision and hearing problems, seizures, and other developmental issues. But microcephaly was thrust into the spotlight by the Zika epidemic, as one of the birth defects that can happen if a pregnant woman is infected with the mosquito-borne virus. Zika increases the chances that a fetus will develop microcephaly and/or other birth defects, particularly if the mother is infected during the first trimester.
New numbers from the Centers for Disease Control and Prevention help quantify that risk: Researchers found that the rate of birth defects in children born to Zika-infected mothers is 20 times higher than normal. The birth defects they looked at included microcephaly as well as eye abnormalities, neural tube defects, and other unspecified central nervous system dysfunctions.
The CDC first looked at the prevalence of these birth defects in populations unaffected by Zika—in Massachusetts and North Carolina in 2013, and in three counties in Atlanta, Georgia, in 2013 and 2014. These regions were chosen because they were running good population-based birth-defect surveillance programs. The prevalence of any of the birth defects in question was 2.86 per 1,000 live births. (Microcephaly specifically was 1.5 per 1,000 live births.)