The seriousness of Zika-infected pregnancies is being underscored in new research from the U.S. Centers for Disease Control and Prevention (CDC).

In the first published research on Zika-related outcomes in the continental U.S. and Hawaii, a preliminary estimate shows that six per cent of babies born to women with possible Zika infection during pregnancy are affected by birth defects.

The defects included microcephaly, a neurological condition in which a baby's skull and brain are undersized, which can lead to severe developmental problems.

To put the findings in context, without the presence of the Zika virus, microcephaly occurs in about seven in 10,000 live births — or less than one-tenth of a per cent.

"This is a really significant increase in a very serious birth defect that has devastating consequences," study author Margaret Honein said in an interview.

"It is an important public health problem. We need to do everything we can do prevent Zika virus in pregnancy. We need to continue to accelerate work on a vaccine to protect pregnant women in the future."

The study, published this week in the Journal of the American Medical Association, was based on data from a U.S. registry started earlier this year to monitor pregnant women with Zika. All were either infected while in an outbreak country or after having sex with someone infected elsewhere.

Zika is mainly spread through mosquito bite.

Out of 442 completed pregnancies, birth defects potentially related to Zika were found in 21 infants and five fetuses that were stillbirths, miscarriages or abortions.

But that rate nearly doubled if the women were infected early in their pregnancy. Among women who were infected in the first three months of pregnancy, birth defects were reported in nine of 85 cases, or 11 per cent of babies. 

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The Zika virus is especially dangerous to pregnant women, as it can cause microcephaly, characterized by unusually small brains and skulls in infants. (Mario Tama/Getty Images)

The rate of birth defects was the same for women who didn't show sign of infection during their pregnancy, as those who did. Since most people who are infected don't show symptoms, the finding emphasizes the importance of screening all pregnant women, Honein said.

Doctors and scientists are still learning about the full spectrum of damage associated with Zika infection. So far, other malformations potentially related to Zika during pregnancy include:

  • Developmental problems.
  • Deafness.
  • Eye abnormalities.
  • Effects on the central nervous system, such as hip dysplasia and clubfoot.

"So many of these infants have just been born. They are relatively young. We have not followed them up yet to understand the full spectrum of adverse things that may go on," Honein said.

Zika and routine prenatal care 

Dr. Aaron Elkin, a Miami-based obstetrician-gynecologist not involved with the research, calls the study pivotal.

"This is a very important study that will reshape future prenatal care in our country and how we deal with patients and counsel them," Elkin said.

In South Florida, where Elkin works, Zika tests are offered to all pregnant women.

"I predict that in the U.S, Zika testing and Zika counselling — even if and when we're going to get the vaccination for it — is going to be a routine part of all prenatal care visits and prenatal care assessments," he said.

Another study published this week in the New England Journal of Medicine found that among 117 infants born to Zika-infected moms in Rio de Janeiro, 42 per cent had "grossly abnormal" clinical or brain imaging findings.

That included four infants with microcephaly, but also cases in which the harms caused were not clear.

Honein said the researchers for the Brazil study used advanced neuroimaging and the clinical significance of those findings aren't yet clear. But the Zika-related birth defects among the 20 pregnant women infected in the first trimester was in the same range as in the U.S. study.

In adults, Zika infections have also been linked to a rare neurological syndrome known as Guillain-Barré, as well as other neurological disorders. All travellers are advised to protect themselves from mosquito bites while in outbreak countries.

As of Dec. 13, there were 421 travel-related cases, 3 sexually transmitted cases and 20 pregnant women with Zika virus reported in Canada. There's been two Zika-related anomalies observed in fetuses or newborns and two other instances were no anomalies were observed, according to the Public Health Agency of Canada. 

PHAC recommends that pregnant women and those planning a pregnancy avoid travel to countries or areas in the U.S. with reported cases of mosquito-borne Zika virus transmission.

The U.S. has reported cases of Zika infection in Florida and Texas, with the most recent investigation taking place in Brownsville, Texas, near the Mexican border.

With files from CBC's Christine Birak and the Associated Press