Babies Exposed to Zika Virus in First Trimester More Likely to Have Birth Defects, Study Says

— -- A first of its kind study of U.S.-born babies exposed to the Zika virus shows higher rates of birth defects for those exposed in the first trimester of pregnancy.

The study, published Monday in the Journal of the American Medical Association (JAMA) is the first time researchers have tried to estimate the likelihood of birth defects, particularly brain abnormalities and microcephaly, developing in a fetus if a pregnant woman is infected with the Zika virus.

The highest rates of birth defects appeared to occur when the pregnant women had the virus in the first trimester. In 85 women who had Zika symptoms or exposure to the virus in the first trimester, 11 percent of the infants had birth defects.

Overall, six percent of infants born to women in the U.S. who were exposed to the Zika virus were found to have birth defects.

"These data highlight the critical importance of primary prevention as recommended by the current CDC guidelines," the authors wrote.

Without the presence of Zika virus, microcephaly occurs in a range between two babies per 10,000 live births, or .0002 percent, and 12 babies per 10,000 live births, or .0012 percent, in the Unites States, according to the CDC.

In this study of preliminary data, researchers from various public health departments and the U.S. Centers for Disease Control and Prevention (CDC) reviewed information on pregnant women likely exposed to the Zika virus in the U.S. and all territories, except Puerto Rico, which is monitored separately.

They identified 442 women, with an average age of 28, who were likely exposed to the Zika virus and who had completed pregnancies between December 2015 and September 2016. Of the 442 completed pregnancies, the researchers determined 26 infants had signs of birth defects, or approximately six percent. This included five fetuses that were found to have birth defects after the pregnancy ended via miscarriage, termination or still birth.

"The more you learn about his virus the nastier it gets," Dr. William Schaffner, an infectious disease expert at Vanderbilt University School of Medicine, told ABC News today.

Twenty-two of the affected infants had brain abnormalities or microcephaly, a defect characterized by an abnormally small head. The four other affected infants had signs of other birth defects including eye and hearing abnormalities. Since the majority of people infected with the virus do not exhibit symptoms and tests to detect the disease in antibodies are not always clear, researchers followed women who were likely exposed to the virus either by traveling to areas where the virus was spreading via mosquitoes or via sexual transmission with an infected person.

Schaffner said the study gives further evidence that any pregnant woman exposed to the virus should be monitored for a long after giving birth since some birth defects may not be evidence immediately after birth.

"Any baby born to a woman with Zika," Schaffner said, "should be followed very, very carefully after birth even if they don't have microcephaly."

As this is one of the first studies to estimate the percentage risk of developing birth defects from the Zika virus, researchers called their findings "preliminary" and called for further long-term study.

In an accompanying editorial Dr. William Muller and Dr. Emily Miller of the Northwestern University Feinberg School of Medicine in Chicago, said even the preliminary findings can help doctors make decisions about patients exposed to the virus.