How strong is the link between Zika and microcephaly?

In April of 2016, the Centers for Disease Control and Prevention (CDC) announced that there is enough evidence to conclude that the Zika virus does cause microcephaly and other severe or fatal birth defects. Women infected with Zika during pregnancy have an increased risk of having a baby with these problems, but not all babies will be affected. The CDC did express worry, however, that major defects such as microcephaly may be the "tip of the iceberg," and that more subtle developmental problems will come to light later. 

The link between the Zika virus and a birth defect called microcephaly (small head) is based on cases in Brazil where pregnant women were infected. The relationship between the maternal illness and microcephaly is not yet clear, if there is a relationship. More studies will be conducted to make this determination.

The absolute cause and effect of Zika virus and microcephaly has not been established. It is not 100% certain that the Zika virus causes microcephaly, but the suspicion is really, really strong. It could take a long time to develop those cause and effect relationships.

In Brazil and other areas where Zika is prevalent, research has shown that a large number of babies born with microcephaly, a birth defect that causes an underdeveloped head and brain, were found to have been infected with Zika. Did it cause the microcephaly? We can’t say with 100% certainty, but based on the number of cases it certainly looks like a potential link exists.