Zika is a mosquito-borne virus that usually causes no symptoms. Yet for some time now, scientists have believed Zika infection during pregnancy can lead to birth defects in the babies. One reason is that the Zika outbreak in Brazil coincided with a microcephaly outbreak. We wanted to see whether there really is a connection between the virus and this and other birth defects or whether there were other factors responsible for the microcephaly outbreak. That is why we gathered information on every birth in Brazil since the beginning of the Zika outbreak. Using statistics we then tested different hypotheses about the cause of the increased rate of microcephaly cases. We found a strong connection between Zika infection early in the pregnancy and this birth defect. The risk is highest during the first two trimesters of the pregnancy. Although Zika virus turned out to cause other birth defects as well, they are a lot rarer as outcomes.