Get breaking news alerts and special reports. The news and stories that matter, delivered weekday mornings.

Three babies have been born with birth defects linked to the Zika virus in the United States, and three more have been lost to miscarriages or aborted because of the birth defects, federal health officials said Thursday.

All of the women were infected with Zika by travel, the Centers for Disease Control and Prevention said. But the CDC also expects localized outbreaks of Zika in the U.S. as mosquito season starts. Puerto Rico already has an epidemic and has also reported at least one Zika-associated birth defect.

Nancy Trinidad, who is 32 weeks pregnant, listens to the explanation of a doctor about how to prevent Zika, Dengue and Chikungunya viruses at a public hospital in San Juan, Puerto Rico, February 3, 2016. REUTERS/Alvin Baez ALVIN BAEZ / Reuters

“The poor birth outcomes reported include those that have been detected in infants infected with Zika before or during birth, including microcephaly, calcium deposits in the brain indicating possible brain damage, excess fluid in the brain cavities and surrounding the brain, absent or poorly formed brain structures, abnormal eye development, or other problems resulting from damage to brain that affects nerves, muscles and bones, such as clubfoot or inflexible joints,” the CDC said in a statement.

Related: CDC Follows Pregnant U.S. Women With Zika

“These numbers are not real time estimates. They will reflect the outcomes of pregnancies reported with any laboratory evidence of possible Zika virus infection as of 12 noon every Thursday the week prior; numbers will be delayed one week.”

"Unfortunately I think it is not surprising."

The CDC says 234 pregnant women in the United States, both residents and visitors, have been diagnosed with Zika. Another 189 cases have been reported in U.S. territories, mostly Puerto Rico.

The Morning Rundown Get a head start on the morning's top stories. This site is protected by recaptcha

Thursday's six cases do not include Puerto Rico or other territories, CDC said.

Related: CDC Confirms Birth Defects in Puerto Rico Fetus

"Unfortunately I think it is not surprising. I think it is consistent with what we have seen in Brazil and Colombia," said the CDC's Dr. Denise Jamieson, who is heading up the agency watch on Zika-affected pregnancies.

"Some, but not all, of the infants have had evidence of Zika virus infection," Jamieson told NBC News.

"What we are reporting is moms who had laboratory evidence of Zika virus infection and the baby had birth defects consistent with Zika virus infection."

Related: Pregnancy and the Zika Virus

CDC and the World Health Organization now say there is no doubt that Zika causes birth defects. They also say there’s no doubt that, like many other infections, it can cause rare neurological complications including Guillain-Barre syndrome.

"What we are reporting is moms who had laboratory evidence of Zika virus infection and the baby had birth defects consistent with Zika virus infection."

“Although these outcomes occurred in pregnancies with laboratory evidence of Zika virus infection, we do not know whether they were caused by Zika virus infection or other factors,” CDC said.

And experts point out that not all birth defects are obvious when a baby is first born. based on what Zika does in a developing fetus -- it seems to head straight for nerve cells -- experts suspect it will also cause vision, hearing and learning problems in some affected babies.

The CDC did not identify the cases in any way but two cases of Zika-affected babies are known about: a Honduran woman who gave birth in New Jersey last month and a woman in Hawaii whose baby was born with microcephaly.

The CDC advises pregnant women, or women who might become pregnant, to stay away from areas where Zika's spreading. And anyone in a Zika-affected area is advised to prevent mosquito bites with clothing, repellent and by staying indoors.

Because Zika can also be sexually transmitted, CDC says any man who may have been infected, even if he doesn't have symptoms, needs to take care not to infect sexual partners who could be pregnant or become pregnant.