Based on data from a past Zika outbreak, researchers estimate that there’s a one-in-100 chance that women infected during the first trimester will give birth to a baby with microcephaly—a birth defect that leaves infants with small and malformed brains and skulls.

There’s still not a definitive link between microcephaly and the mosquito-borne virus, which is currently blazing through Central and South America. But the data coming in has only strengthened the connection. For instance, a recent study found that Zika virus can kill off developing brain cells.

With the new estimate on the rate of microcephaly in Zika infections from an outbreak in French Polynesia starting in 2013, researchers are relieved that it appears the birth defect is relatively rare.

“It means you have a 99 percent chance of having a normal baby,” Dr. Laura Rodrigues, a professor of infectious disease epidemiology at the London School of Hygiene & Tropical Medicine, told The New York Times.

By contrast, birth defects from other in utero viral infections can have far steeper risks. Pregnant women infected with the rubella virus, for example, have at least a 38 percent chance of having a baby with congenital rubella syndrome, which can lead to heart disease, developmental delays, and hearing loss.

To come up with an estimate for Zika, researchers sifted through medical records from the outbreak of Zika in French Polynesia between 2013 and 2015, which sickened 66 percent of the 270,000 residents. The researchers also established the baseline of microcephaly in the population, which was two cases in 10,000 births. During the outbreak, there were eight total cases, the researchers reported this week in The Lancet.

While the rate is relatively low, health experts are not sure if it would be the same in different populations. Even if it is, the virus is still spreading to millions throughout the dozens of countries.

“If you apply a one percent risk to a large number of women, it’s still a large public health problem,” said Simon Cauchemez, the study’s lead author and the director of mathematical modeling for the infectious diseases unit at the Pasteur Institute in Paris.

The Lancet, 2015. DOI: 10.1016/S0140-6736(16)00651-6 (About DOIs).