Evidence supporting the link between the Zika virus and microcephaly has been found in a retrospective study of the outbreak in French Polynesia between October, 2013 and April, 2014.

Researchers examined medical records to find cases of microcephaly between September, 2013 and July, 2015. They found eight cases of microcephaly during this time, seven of which occurred between March 1 and July 10, 2014.

The timing of these cases is best explained by a period of risk during the first trimester, according to the authors. Using their research they estimate the risk of microcephaly is about 1 for every 100 women infected with Zika during the first trimester. However they say determining how Zika may cause microcephaly needs further research.

Original research paper published in The Lancet on March 15, 2015.

Names and affiliations of selected authors

Simon Cauchemez, Mathematical Modelling of Infectious Diseases Unit, Institut Pasteur, Paris, France