US health officials have confirmed the Zika virus does cause microcephaly, a severe birth defect that causes children to be born with abnormally small heads and brain damage.
The World Health Organization declared a global emergency after the mosquito-borne virus was linked to thousands of cases in Brazil.
The US Centers for Disease Control and Prevention (CDC) says the evidence is now overwhelming.
"What we're learning is that they have a severe form of microcephaly that is often times associated with other problems in the brain that can be seen on imaging, on CT scans or MRIs, that make us really concerned," says Dr Sonja Rasmussen.
"It is an unprecedented thing -- we are not aware of other infections that are spread by mosquitoes that can cause birth defects."
There have been 80 confirmed cases of Zika in New Zealand, but all but one have been infected overseas.
The virus can also be transmitted through sexual contact.
The Ministry of Health recommends women who are pregnant or plan to become pregnant in the near future consider delaying travel to areas with the Zika virus present.
"Typically we find out the tip of the iceberg first and then we learn more -- sometimes it takes years to understand the full spectrum," says Dr Rasmussen.
Fast-tracked research is under way to find out more about the virus and to develop a vaccine, but that could still be years away.
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