A woman infected by the Zika virus during the first three months of pregnancy faces a one-in-100 chance her child will suffer severe brain damage, according to a study released Wednesday.

Zika increases the risk of microcephaly -- an otherwise rare condition that results in an abnormally small head -- by fifty-fold, the researchers calculated.

"The first trimester is the most critical," lead author Simon Cauchemez, a scientist at the Institut Pasteur in Paris, told AFP.