Meera Senthilingam, CNN - Prince Harry and Meghan, the Duchess of Sussex, are expecting their first child, the UK's royal family announced Monday as the couple arrived in Australia for their first tour overseas.

The tour will include visits to Tonga and Fiji, which both carry risk of the Zika virus, a cause of birth defects .

Kensington Palace considered medical issues before Meghan left for her trip, a royal source said Monday, but the couple did not make any changes to their schedule.

The World Health Organization and the US Centers for Disease Control advise against travel to both islands while pregnant. The UK Foreign Office suggests women consider postponing non-essential travel until after pregnancy.

The CDC classifies Fiji and Tonga as being "areas with risk of Zika infection." WHO places Tonga in risk category 1, an area with new introduction or ongoing transmission, and Fiji in category 2, an area of evidence of the virus circulating before 2015 or ongoing transmission.

Infection with Zika during pregnancy has been linked to miscarriage and stillbirth, and can cause babies to be born with microcephaly (a smaller than usual head size indicating incomplete brain development), vision problems or neurological defects. The CDC estimates 1 in 7 babies exposed to Zika has health problems.

The virus is transmitted primarily through the bite of infected Aedes mosquitoes. It can also spread through unprotected sex with an infected person, even if that person is not showing symptoms. It could also spread through blood transfusions.

Zika virus can spread from a pregnant woman to her fetus, but the likelihood of an infection affecting pregnancy or causing birth defects is unknown, according to the CDC. The full range of health effects during pregnancy is also unknown, the agency says.

If pregnant women find themselves in a region affected by Zika, experts advise they take steps to prevent mosquito bites -- such as wearing covered clothing and using insect repellants and mosquito nets -- and use condoms or abstain during the entire pregnancy.

Most people infected with the Zika virus won't have symptoms. Symptoms typically appear a few days to a week after exposure, although the precise incubation period is not known, according to the CDC.

Fever, rash, joint pain and conjunctivitis (pink eye) are the most common symptoms. Some patients may also experience muscle pain or headaches. The major concern remains among pregnant women.

As of March 2018, the CDC estimated at least 90 countries and territories had active Zika virus transmissions.

People in Brazil and other parts of South and Central America became infected when Zika first appeared in the Western Hemisphere in 2015.

No vaccine is available, but clinical trials are being conducted.

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