Women infected with the Zika virus during the first trimester of pregnancy face a roughly 1 in 100 chance that their unborn child will develop the serious birth defect microcephaly. This is according to a new study published earlier this week in The Lancet, which is the first to give a concrete estimate of the risks associated with the virus.

The study itself looked at the prevalence of birth defects during a recent outbreak of the Zika virus in French Polynesia. The region has a small population — around 270,000 — making it easy for researchers to collect all the relevant cases. During the outbreak between 2013 and 2015 there were eight cases of microcephaly, with seven of these occurring over the last four months of this period. This clustering, say the researchers, strongly supports the proposed association between the virus and the birth defect, although the exact mechanism that's at work is still unknown.

The researchers used this data and statistical modeling to create the estimated risks associated with Zika. However, the study does not take into account other abnormalities that may be associated with the virus, and its authors warn that the risk may differ from outbreak to outbreak. Speaking to The Wall Street Journal, the study's co-author Arnaud Fontanet said: "The one percent we describe here is not the end of the story."