Even worse, the data show that birth defects can occur if a woman is infected in any stage of pregnancy.
The mosquito-borne disease, which is also sexually transmitted, is of greatest concern to pregnant women and those trying to become pregnant and their partners because of the devastating consequences it can have on a fetus.
"We are still learning about the full range of defects, brain abnormalities, vision problems and other brain issues," Acting CDC Director Dr. Anne Schuchat said. "But this data shows that Zika is associated with risk for the fetus when the infection is identified later in pregnancy."
The CDC data, which represent the "largest number of completed pregnancies with laboratory confirmation of Zika virus infection to date," were collected from the US territories of Puerto Rico, American Samoa, the Federated States of Micronesia, the Republic of Marshall Islands and the US Virgin Islands. This is the first such CDC report about how the virus has affected pregnant women and their babies in those areas.
"Some (babies) have seizures, while others have little to no control over their limbs and can't reach out to touch things around them," Schuchat said. "Some have significant feeding difficulties and have trouble swallowing or even breathing while feeding. Some babies cry constantly and are often inconsolable no matter what their caregiver does to soothe them."
A total of 3,900 women with possible Zika infections were followed between January 1, 2016, and April 25, 2017. Of those, 2,549 delivered babies in that time. Of the women who carried to term,1,508 had confirmed infections, and 122 of those had babies obviously affected by the Zika virus.
Although 8% of the birth defects occurred when women were infected during the first trimester, the numbers were not much better for those infected later in their pregnancies. Five percent of women diagnosed in their second trimesters and 4% of those in the third trimester also gave birth to babies suffering the devastating effects of Zika.
"We have also learned that the effects of Zika during pregnancy are not always obvious at birth," Schuchat said, adding that babies may initially look normal but develop signs of Zika complications such as small heads or vision and hearing problems. "That's why identification of and followup and care of babies with possible Zika-virus infection is so crucial."
This is the first report, the CDC said, with sufficient numbers of Zika virus infections identified during the second and third trimesters of pregnancy to develop preliminary estimates of risk for each trimester.
Last week, the governor of Puerto Rico announced that the Zika "epidemic" was over, citing only 10 new cases since the end of April. Does the CDC agree with that assessment?
"We do agree the disease went up and has come down," Schuchat said, but she stressed that the CDC was not changing its travel guidance to the territory. "We are pleased that the incidence of new cases in Puerto Rico is low at this point, but we continue to urge pregnant women not to travel to areas where Zika is still spreading, and that includes Puerto Rico."
As of May 20, the most recent data available, the Puerto Rico Department of Health reported
(PDF) 40,330 confirmed cases of the Zika virus since the outbreak began last year. Four hundred twenty-two of those infected have been hospitalized, and five have died. Among those cases, there have been 52 Zika-related cases of Guillain-Barre syndrome, a type of temporary paralysis that is caused by viruses, including Zika. The Puerto Rico Department of Health has reported only 38 cases of Zika-related birth defects.
The CDC has also been tracking mainland US births and found that one in 10, or 5% of, women with confirmed Zika infections had a fetus or baby with Zika-related complications in 2016.
All of the 51 women in the US who contracted Zika while pregnant did so during visits to 16 areas where the virus is circulating: Belize, Barbados, Brazil, Cape Verde, Colombia, the Dominican Republic, El Salvador, Guatemala, Guyana, Haiti, Honduras, Jamaica, Mexico, Puerto Rico, the Marshall Islands and Venezuela.
Despite a reduction in new cases in some areas, Zika is still prevalent around the globe. The CDC reminds women who are pregnant or trying to become pregnant to avoid travel to areas where the virus is circulating
and to follow recommendations for preventing sexual transmission of the virus if their partner has traveled to such an area.
"Zika hasn't gone away," Schuchat added. "While the number of new infections have recently decreased, the risk is still high for women and their families."