The problems include a rare and severe birth defect known as microcephaly, which is characterized by abnormally small heads and often underdeveloped brains, as well as an array of other fetal abnormalities.
Other complications include calcium deposits in the brain indicating possible brain damage; excess fluid in the brain cavities and surrounding the brain; missing or poorly formed brain structures; abnormal eye development; and other problems resulting from damage to the brain that affects nerves, muscles and bones, such as clubfoot or inflexible joints, officials said.
The problems are consistent with the damage caused by Zika virus infections reported in other countries with Zika outbreaks, including Brazil and Colombia.
The information released Thursday provides additional details about the impact of the virus on pregnant women in the United States. The agency is monitoring 234 pregnant women with Zika on the U.S. mainland who contracted the virus through travel or an infected partner; another 189 are being monitored in Puerto Rico and other U.S. territories.
Widespread local transmission already is occurring in Puerto Rico, and officials expect that about 20 percent of the island’s 3.5 million residents could become infected.
In the coming weeks, the CDC will begin reporting Zika-linked pregnancy outcomes in the U.S. territories.
In February, the CDC detailed what happened to nine pregnant women in the United States who had been infected with Zika. Two of those women chose to have abortions; two others suffered miscarriages; one gave birth to an infant with serious birth defects; and two others delivered healthy infants. At the time, two were still pregnant. Officials said the lost or terminated pregnancies reported in February did not necessarily involve Zika-related birth defects.
Last month, the CDC announced it was monitoring about 300 pregnant women with possible Zika infection but declined to provide details about the pregnancy outcomes. Since then, the agency has received many requests for that information from federal, state and local health officials, as well as from health-care providers, pregnant women and their families.
Earlier in the year, there were only a few known cases of microcephaly in the United States. With the information about these six cases, "we feel we can report in aggregate without reporting specifics and where the outcomes are occurring," said Denise Jamieson, the chief of CDC's women's health and fertility branch, in an interview.
A baby with microcephaly was born to a Zika-infected woman in Hawaii in January. The mother had lived in Brazil last year and probably was infected by a mosquito early in her pregnancy, the Hawaii state health department has said.
Last month, a baby with severe microcephaly and other birth defects was born in New Jersey to a Honduran woman. The girl was the first to be born to a Zika-infected mother on the U.S. mainland. Not only does the baby have microcephaly, doctors said, but she is also suffering from intestinal and eye abnormalities.
In the case of a District woman, Zika successfully hid through nearly half her pregnancy, its damage to her fetus not showing despite a series of early ultrasounds. The The 33-year-old woman still tested positive for Zika 10 weeks after she likely was infected during a trip to Guatemala – far beyond what scientists have thought is the case. In early February, the woman terminated the pregnancy.
The CDC said the agency is providing additional information now so people will have the most up-to-date information about pregnancy risks associated with Zika virus infection during pregnancy.
Recent studies have added to the evidence that Zika-infected pregnant women face potentially grave consequences.
Researchers studying the Zika outbreak in Colombia reported Wednesday that even pregnant women who don't show symptoms of infection are at risk of having babies with microcephaly.
In Brazil, pregnant women infected with Zika during their first trimester face as high as a 13 percent chance that their fetus will develop microcephaly, researchers reported last month.
"What we're seeing is a very consistent pattern underscoring the fact that Zika causes microcephaly and other severe brain abnormalities," Jamieson said. "This highlights the importance of preventing unintended pregnancies, avoiding mosquito bites and for pregnant women to avoid traveling to areas with ongoing Zika virus transmission."
U.S. officials are anticipating local cases of Zika infection because the mosquito that is the primary vector, Aedes aegypti, is found throughout the South as well as in parts of the Southwest and even Midwest. But they do not expect an explosion of cases, as has been seen in Puerto Rico and some countries in the Caribbean and South America.
Despite dire warnings from public health officials and experts about the spread of the virus this summer, Congress has balked at approving the Obama administration’s $1.9 billion request for Zika funding. The Senate approved $1.1 billion in funding in May. The House passed legislation that would provide $622 million, which would be drawn from money already set aside for Ebola programs.
In April, the administration redirected more than $500 million from Ebola funds to get geared up for Zika, but CDC Director Tom Frieden and other experts say Congress needs to act immediately so expanded measures can be put into place and be most effective when cases surface.
In particular, Frieden has said the funds are needed to allow more people, especially pregnant women, to more easily get tested and get a prompt result, and to conduct ongoing studies about Zika's long-term developmental impact on children.