A total of nine pregnant travellers have been found in lab tests to have Zika infections, the U.S. Centers for Disease Control and Prevention says, while reminding people to take precautions against mosquito bites and potential sexual transmission of the virus.

Since August, the CDC said it has tested 257 pregnant women for Zika. Eight were positive for the virus and a state lab confirmed a ninth.

Two pregnancies ended in miscarriage, but it's not clear if the Zika infection contributed. Two women had abortions and two are continuing without reported complications.

All nine women experienced symptoms of Zika infections such as fever, rash, red eyes or joint pain. 

The Zika virus is epidemic in Latin America and the Caribbean. In previous outbreaks, the virus caused mild illness or no symptoms in most people.

A possible association between Zika infection in pregnancy and microcephaly — babies with small heads — and incomplete brain development has public health authorities warning pregnant women to avoid travel to Zika-affected areas.

Men who live in or travel to areas where Zika is spreading are advised to abstain from sex or use condoms consistently and properly with their pregnant partners.

Studies to determine whether Zika causes microcephaly are a priority. The World Health Organization has said that could take six months, through studies such as following pregnant women in Colombia.

In the meantime, scientists are publishing details on pregnancy outcomes as they gather and analyze them.

In one small study published in CDC's weekly Morbidity and Mortality Weekly Report, Zika virus infection during pregnancy was associated with a range of outcomes, including early pregnancy losses, microcephaly and apparently healthy infants.

"We did not expect to see these brain abnormalities in this small case series of U.S. pregnant travellers," Dr. Denise Jamieson, a birth defects expert serving on CDC's Zika virus response team, told reporters. 

CDC director Dr. Thomas Frieden said scientists at the agency's lab identified Zika virus in placental tissues. 

"That is suggestive that Zika may have caused the miscarriage," Frieden said, noting that 10 per cent to 20 per cent of  pregnancies end in miscarriage. 

A second report published Friday by the CDC focused on 14 instances of suspected sexual transmission among women who had sexual contact with a male who had travelled to an area of Zika virus transmission. The CDC received the reports between Feb. 6 and 22.

The CDC director told reporters they expected Zika cases to occur among travellers returning to the U.S.

"We did not, however, anticipate that we would see this many sexually transmitted cases of Zika," Frieden said in a teleconference.

All of the sexual transmissions were linked to men who were showing symptoms or had just recovered. Studies are underway to see how long the virus can persist in semen.

Several of the women who became infected through sexual transmission were pregnant. No further details were provided to protect patient confidentiality. 

The CDC is also investigating another 10 cases of suspected Zika infection in pregnant travellers

With files from The Associated Press and Reuters