MIAMI, United States (AFP) – US health authorities Friday described the cases of nine pregnant women who contracted the Zika virus while traveling, two of whom chose abortion and one who gave birth to a baby with microcephaly.

The women’s identities were not released by the US Centers for Disease Control and Prevention, which described them in a report covering the time period of August 2015 to February 2016.

Zika, a mainly mosquito-borne virus, has been linked to a spike in birth defects in Brazil, where thousands of babies have been born with unusually small heads since last year.

Officials are rushing to figure out if Zika is the cause of the irreversible damage, known as microcephaly.

CDC chief Tom Frieden cautioned that much more research on a large number of people is needed to fully understand the risk posed by Zika virus infection in pregnancy.

But the details of the nine cases suggested that adverse effects were more likely when the women were infected early on.

Of the six women who tested positive for Zika during the first trimester, one pregnancy is ongoing.

One woman gave birth to a baby with microcephaly — a case that has been previously reported by state health authorities. She is believed to have been infected while traveling to Brazil.

Two of the women miscarried, and two elected to terminate their pregnancies.

Severe brain defects were documented during an ultrasound and MRI scan at 20 weeks gestation in one of the cases in which the woman chose an abortion.

Details on the second abortion case were not released, and the CDC declined to say whether or not both the women made their decisions specifically because of Zika.

Two of the nine women tested positive for Zika later in their pregnancies, during the second trimester.

One delivered a healthy baby and the other is continuing her pregnancy.

The ninth woman “reported symptoms of Zika virus infection in the third trimester of pregnancy, and she delivered a healthy infant,” said the CDC.

All the women reported common symptoms of Zika virus infection — including fever, rash and or red eyes — and all were confirmed in lab tests to have Zika virus infections.

There is no vaccine or treatment for Zika. Health authorities have urged people traveling to or living in the more than 20 affected areas of Latin America and the Caribbean to avoid mosquito bites if possible, and to choose abstinence or use condoms regularly to avoid passing the virus to a partner.

The CDC said 10 additional reports of Zika virus disease among pregnant women are currently under investigation.