By Siouxsie Wiles 16/03/2016

New research which looked at the data from the 2013-2014 outbreak of Zika in French Polynesia, estimates that the risk of microcephaly is about 1 for every 100 women infected with the Zika virus during the first trimester of pregnancy.

Zika is the virus spread by mosquitoes (and more rarely by sexual transmission) that is currently causing concern in over 30 countries and territories across the Americas and Pacific because of a potential link between infection during pregnancy and babies being born with smaller heads and brains (known as microcephaly). The last few weeks has seen a flurry of papers and reports strengthening that link. Now a paper has been published in the journal The Lancet which looks back at data from a previous outbreak of Zika, which happened in French Polynesia (1). The outbreak began in October 2013, peaking in December and ending the following April. More than 31,000 people visited their doctor with suspected Zika virus infection. During the outbreak, 8 cases of microcephaly were identified, 7 of them during a four month period around the end of the outbreak.  Five of the 8 pregnancies were terminated.

Using researchers built a mathematical model to estimate the expected number of microcephaly cases using data on the total number of cases of microcephaly, the weekly number of doctors visits for suspected Zika virus infection, blood tests confirming the presence of Zika virus antibodies taken post-outbreak, and the total number of births during the outbreak. Using their model, the researchers were estimate that the risk of microcephaly is 95 in 10,000 women (approximately 1 in 100) infected with Zika virus in the first trimester of pregnancy. The normal risk is about 2 per 10,000.


  1. Cauchemez et al. Association between Zika virus and microcephaly in French Polynesia, 2013–15: a retrospective study. The Lancet.

Featured image: Flickr CC, Douglas Porter.