These estimates are higher than what U.S. health officials have previously reported and underscore the serious risk for birth defects posed by Zika virus infection during pregnancy. With warm weather, a new mosquito season and summer travel approaching, prevention is crucial to protecting the health of mothers and babies, said Anne Schuchat, the CDC's acting director.

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It may seem like Zika is last year's problem, she said, but that is not true, and pregnant women, their male partners, and clinicians can't be complacent.

"Don't let this outbreak be your family's heartbreak," she said Tuesday during a telephone briefing with reporters.

Most infections are spread by mosquitoes, but the virus also can be transmitted through sex and bodily fluids.

Previous studies of Zika-associated birth defects were based on data from a small number of completed pregnancies. This new report analyzed data from nearly 1,000 pregnant women in 44 states who completed their pregnancies in 2016 and had some evidence of a Zika infection. Most were exposed during travel to one of the dozens of Zika-affected countries.

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Of the 972 completed pregnancies, 250 women had confirmed infections. Of those, 24 pregnancies — about 10 percent — resulted in a fetus or baby with birth defects.

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Screening for Zika is difficult because many infected people don’t have symptoms that might motivate them to get tested, and the longer the time lag after infection, the more complicated diagnosis becomes.

Even more disturbing, the risk for birth defects is almost certainly higher than the study suggests, because three-fourths of the infants born to mothers with some evidence of Zika infection did not receive the recommended brain imaging after birth. The link between Zika and birth defects was first noticed because the infection can cause microcephaly, or an abnormally small head. Subsequent research showed that babies may appear healthy at birth, with a normal head size, but have underlying brain abnormalities. Of 895 live births, only 221 babies received any kind of neuroimaging.

“It’s really key for these babies to have a head ultrasound or CT scan to look for abnormalities that may not be apparent at birth,” Margaret Honein, chief of the birth defects branch at the CDC and author of the report, said in an interview. “Knowing that not all these babies are receiving brain imaging, this may be significantly underestimating the complete number of infants with birth defects.”

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Honein said she didn’t know why so many babies were not receiving the recommended brain evaluations. The reasons are likely to be complex, she said. All health-care providers, especially obstetricians and pediatricians, need to improve monitoring of these infants. The CDC is monitoring all pregnant women in the United States with any evidence of Zika infection; the agency is receiving 30 to 40 new reports of such pregnancies every week, she said.

Although microcephaly has been the most high-profile birth defect associated with Zika infections, congenital Zika syndrome is associated with a broad range of symptoms. They include eye abnormalities, hearing loss and problems moving limbs.

As a result, some babies have seizures, others have little to no control over their arms and legs, and can't freely reach out to touch things around them because of constricted joints, Schuchat said during a briefing with reporters. Some babies are not reaching their typical developmental milestones, like sitting up. Others have trouble swallowing or even breathing while feeding. Some babies are often inconsolable no matter what their caregiver does to soothe them.

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"We’ve seen that these circumstances are just heartbreaking for families and clinicians," she said.

The more than 5,000 Zika cases reported in the continental United States and Hawaii include more than 1,600 pregnant women with evidence of Zika infection.

The cost for treating an infant with microcephaly is estimated at nearly $4 million, Schuchat said. For those who survive into adulthood, the cost could be up to $10 million.

The information released Tuesday provides an analysis of what has happened to the largest number of Zika-infected completed pregnancies. The information comes from state officials reporting to the U.S. Zika Pregnancy Registry from Jan. 15 to Dec. 27, 2016, in the 50 states, Washington, D.C., and all U.S. territories except Puerto Rico. Pregnancies in Puerto Rico, which has by far the largest number of Zika infections of anywhere in the United States, are being tracked separately.

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Despite the differences in methodology and case definition, the CDC findings are consistent with reports from other countries about Zika-related birth defects, Honein said.

Researchers don’t know the full impact of a congenital Zika infection, and for that reason, they want to have the babies followed and evaluated for the first year or two, she said.

It’s unclear whether the CDC will have the funding to conduct follow-up evaluations of the babies. The agency, which is part of the Department of Health and Human Services, does not yet have a budget for the fiscal year that ends in October, or for the next fiscal year. The Trump administration has proposed cutting next year’s HHS budget by about 18 percent. It’s not clear whether those cuts would be applied across the board.

“We don’t yet know what the final funding amounts will be,” CDC spokesman Bertram Kelly said.

But the CDC and HHS are “committed to improving the health and well-being of the American people and look forward to continuing to work on long-term solutions that will further enhance our ability to positively and productively serve the American people,” he said.

Among the report’s key findings: