About 1 in 10 pregnant women in the U.S. who were infected with the Zika virus gave birth to babies with birth defects or discovered birth defects during pregnancy, according to a study released Tuesday by the Centers for Disease Control and Prevention.
The proportion is significantly higher than officials initially believed. A previous report observed complications of the Zika virus in 2.9 per 1,000 live births in the U.S., or 30 times lower than the latest report suggests.
The findings, published in the agency’s Vital Signs report, used data from 1,297 women in 44 states who showed some signs of having been infected with Zika from Jan. 15 to Dec. 27 of last year, though it uses final data from only 972 women, because they gave birth to a live or stillborn baby or had an abortion or miscarriage during that period.
Birth defects were observed among 5 percent of those believed to have been infected based on evidence such as a rash or fever symptoms tied to travel dates, and among 10 percent of those who had lab tests showing positive results for Zika.
The proportion of fetuses or babies who developed signs of birth defects was highest among women who tested positive for the virus during the first trimester, at 15 percent. The data adds to previous evidence supporting scientists’ theory that risks of developing birth defects are higher when a woman is infected earlier in a pregnancy.
The report could be understating the actual incidence of Zika-linked birth defects. The virus is often difficult to detect because roughly 80 percent of adults have no symptoms, but it is particularly dangerous for pregnant women because it can spread to a developing fetus, causing the birth defect microcephaly, which results in a baby being born with an unusually small head and with possible brain damage. As scientists have studied the virus, they have found additional deformities in some babies, including curving limbs, as well as signs of hearing or vision loss.
Health providers are unable to determine whether a fetus has developed microcephaly until late in the second trimester or early in the third. The CDC’s report refers to “pregnancy losses” to characterize abortions, miscarriages or stillbirths, which comprised 77 of the 972 cases.
Most women became infected by visiting a country where Zika was circulating among Aedes aegypti mosquitoes, though the virus can also be transmitted sexually. The CDC issued guidance last year about mosquito prevention measures and recommended that women who were pregnant avoid traveling to countries where the virus was circulating among mosquitoes. The CDC also issued guidance urging patients to use condoms when they have sex or to abstain from sex for the duration of a pregnancy.
The CDC reminded healthcare providers of this guidance Tuesday as the country nears the summer, the time of year when mosquitoes become more prominent. The virus circulated in parts of Miami last year, though most infections occurred following travel to parts of Latin America, the Caribbean and Puerto Rico.
But the report did not use pregnancy infection data from Puerto Rico, where Zika infection rates surged to 37,000 and where the U.S. territory is collecting its own pregnancy registry.
In a call with reporters ahead of the study’s release, officials noted that a quarter of babies who had possible issues related to Zika did not receive brain imaging after birth, as the agency recommends. Scientists have noted that sometimes microcephaly isn’t noticed until later and that neurological problems can still occur among babies whose heads appear structurally normal.
Margaret Honein, a co-author of the report and chief of the CDC’s Birth Defects Branch, warned during a call with reporters that the report “might significantly underestimate the impact of Zika.”
