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Health officials said Friday they have confirmed nine cases of Zika virus among pregnant women in the U.S., and say they are investigating 10 more suspected cases.

The numbers have surprised the Centers for Disease Control and Prevention, which has been on the lookout for pregnant travelers affected by Zika. Five of the nine affected babies or fetuses have either miscarried or shown evidence of birth defects.

Two of the pregnancies with the virus, which is suspected of causing severe birth defects, have ended in miscarriage, two more were aborted, and one baby was born with severe microcephaly. Two babies were born healthy, the CDC reported, and two women are still pregnant with apparently healthy babies.

"We did not expect to see these brain abnormalities in this small case series of U.S. pregnant travelers," Dr. Denise Jamieson, who's helping lead CDC's Zika response, told reporters in a conference call. "It is unexpected and greater than what we would have expected."

And CDC officials also said they'd been surprised at how many sexual transmissions of the virus have been seen so far.

"We did not expect to see these brain abnormalities in this small case series of U.S. pregnant travelers."

Brazil has reported more than 5,600 cases of microcephaly, a severe birth brain defect, since October, the World Health Organization announced separately Friday. WHO said it has not yet been proven Zika is causing any of them, but says the evidence is strengthening.

And both WHO and CDC said it was becoming increasingly apparent that Zika could cause Guillain-Barre syndrome, a usually rare complication of infections of various kinds.

All of the 19 pregnant women eyed by CDC were U.S. residents who had traveled to Zika-affected areas, the CDC said. With more than 30 million Americans traveling to Central and South America and the Caribbean every year, the CDC expects many travelers to bring home infections.

The case of the baby born with microcephaly had been reported by state health officials in Hawaii but CDC said it would not identify any of the women by state.

"Approximately half a million pregnant women are esti­mated to travel to the United States annually from the 32 Zika-affected countries and U.S. territories with active transmission of Zika virus," the CDC said.

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These nine early cases may indicate that the first trimester of pregnancy may be the most dangerous time for a woman to become infected with the virus, although it is not 100 percent certain yet that Zika is causing the birth defects.

Doctors say the evidence is building, with several miscarried or aborted fetuses showing evidence of the virus in their brains, along with severe damage to the brain tissue.

CDC has warned pregnant women to avoid travel to Zika-affected areas.

“All nine women reported at least one of the four most commonly observed symptoms (fever, rash, conjunctivitis, or arthralgia), all women reported rash, and all but one woman had at least two symptoms,” CDC said. Arthralgia is a medical term for joint pain.

“Among two women with Zika virus infection who had symptoms during the second trimester of pregnancy, one apparently healthy infant has been born and one pregnancy is continuing. One pregnant woman reported symptoms of Zika virus infection in the third trimester of pregnancy, and she delivered a healthy infant,” CDC added.

"For the American public, the bottom line hasn’t changed," CDC Director Dr. Tom Frieden told reporters. "If you are pregnant, avoid travel to a place where Zika is spreading." Women in affected U.S. territories, such as American Samoa and Puerto Rico, should protect themselves from mosquito bites as well as they can.

CDC, in collaboration with the CDC Foundation, has assembled more than 5,000 Zika prevention kits for pregnant women in Puerto Rico, with an additional 45,000 planned. If Zika is reported in the United States the kits will also be distributed to pregnant women in affected areas. The ZPK provides pregnant women with an initial supply of prevention tools including repellent, products that kill mosquito larvae, and condoms to prevent sexual transmission. CDC

CDC said it was distributing Zika prevention kits for pregnant women in Puerto Rico.

"There is the potential for hundreds of thousands of cases of Zika in Puerto Rico and unfortunately, tragically, affected pregnancies as well," Frieden said.

CDC has also cautioned men coming back from affected areas to be careful, as it’s becoming clear the virus can be transmitted sexually. CDC reported 14 suspected or confirmed cases of sexual transmission in the U.S. but says two of those have now turned out not to be sexually transmitted Zika infections.

"We know that, at least while men are symptomatic, they can spread Zika to their sexual partners."

CDC gave some details of some of the cases, including a pregnant woman in her 30s who traveled to a Zika-affected area early in her pregnancy.

“One day after returning from travel, she developed fever, eye pain, and myalgia (muscle ache). The next day, she developed a rash,” the CDC team said. A blood test confirmed she had Zika.

When she was 20 weeks pregnant, am ultrasound showed the fetus had severe brain damage. An amniocentesis test showed virus in the amniotic fluid, which indicates the fetus was infected.

“After discussion with her health care providers, the patient elected to terminate her pregnancy,” CDC said.

CDC said it was rushing tests out to states so that pregnant women who suspect they are infected can be tested for Zika. So far, it's tested 257 pregnant women and just 3 percent of them have turned out to have had Zika.

Related: Did Zika Cost This Woman Her Baby?

CDC also detailed several cases of sexual transmission of Zika. All involved a man who had traveled to a Zika-affected area and had unprotected sex with a woman while he had symptoms, such as a rash or fever.

"We know that, at least while men are symptomatic, they can spread Zika to their sexual partners," Frieden said.

"Health care providers should now consider any person who has had condomless sex (i.e., vaginal inter­course, anal intercourse, or fellatio) with a male partner who has traveled to an area of ongoing Zika virus transmission and who has had symptoms of Zika virus disease during travel or within two weeks of return as potentially exposed," the CDC advised.

Zika's a mosquito-borne virus, spread as female Aedes mosquitos sip blood from one person after another, often in the same room. Other viruses are spread this way, too: yellow fever, dengue, West Nile and chikungunya.

With Zika, right now the only way the virus is getting to unaffected countries like the U.S. is in the bodies of travelers. So the cases of sexual transmission stand out. CDC says 30 million to 40 million Americans fly to Latin America and the Caribbean every year and even more go by land.