WEDNESDAY, June 15, 2016 (HealthDay News) -- The danger of Zika-related birth defects may be confined to maternal infections that occur during the first two trimesters of a pregnancy, a new study suggests.
Colombian and U.S. researchers studied almost 12,000 pregnancies occurring in 2015 among women in Colombia, a country that is endemic for the mosquito that spreads the Zika virus.
The study detected no cases of infant abnormalities among women who contracted Zika during the last three months of their pregnancy, the researchers said.
They stressed that at the time of the study's publication, 10 percent of the 1,850 women infected late in pregnancy had not yet given birth -- so the data remains incomplete and "preliminary."
Still, data on the other 90 percent of women suggest that "maternal infection with the Zika virus during the third trimester of pregnancy is not linked to structural abnormalities in the fetus," the researchers concluded.
Those Zika-linked abnormalities most commonly involve microcephaly -- a devastating birth defect where infants are born with abnormally small heads and associated neurological issues.
The study was published June 15 in the New England Journal of Medicine and led by Margaret Honein, of the U.S. Centers for Disease Control and Prevention.
In other Zika news, the CDC on Tuesday announced a plan to respond in a targeted and rapid way to any outbreaks of Zika infection in the United States, as the summer mosquito season heats up.
According to the Associated Press, if local infections occur, the affected state can reach out to expert teams from the CDC that will travel to the area. Part of the plan also involves detailed steps on destroying mosquitoes and their breeding sites within 150 yards of the infected person's property.
Such efforts would continue for at least 45 days after the last reported Zika illness in the area, the AP said.
The vast majority of Zika infections so far have occurred in Latin America, and Brazil has been the epicenter with an estimated 5,000 cases of microcephaly. There have been no reports of Zika-induced microcephaly contracted in the United States, although Zika is now circulating in Puerto Rico, the U.S. Virgin Islands and American Samoa.
U.S. health officials have said they expect to see mild Zika outbreaks in Gulf Coast states such as Florida, Louisiana and Texas as mosquito season picks up.
Last Friday, the World Health Organization issued a recommendation that couples who are trying to have children and live in Zika-affected areas should consider delaying pregnancy to avoid having babies born with birth defects.
In the United States, the CDC has refrained from recommending that couples delay pregnancy in Zika-affected areas. However, Puerto Rico's health secretary has issued advice that is similar to the new World Health Organization (WHO) guideline, The New York Times reported.
Mosquito bites remain the most common way Zika is spread, but transmission of the virus through sex is more common than previously thought, WHO officials have said.
Although the new WHO guideline doesn't specify how long couples should delay pregnancy, the recommendation "means delaying until we have more answers, more evidence, more science," WHO spokeswoman Nyka Alexander told the Times.
Last month, U.S. health officials reported that the number of pregnant women in the United States infected with the Zika virus had tripled because cases were now being counted in a more comprehensive way.
So far, an estimated 280 infected women are being followed in the United States and its territories, according to two registries that have been created by the CDC.
Previously, only cases of pregnant women who had Zika-related symptoms or pregnancy complications were being tallied, CDC officials said. But recently published reports have found that some pregnant women show no symptoms of Zika infection, yet still give birth to babies with microcephaly.
Women of child-bearing age who live in an active Zika region should protect themselves from mosquito bites by wearing long-sleeved shirts and long pants, using mosquito repellent when outside, and staying indoors as much as possible, according to the CDC.
President Barack Obama has asked Congress to allocate $1.9 billion to combat the Zika threat, but lawmakers have yet to agree on a spending package.
Visit the U.S. Centers for Disease Control and Prevention for more on the Zika virus.
This Q&A will tell you what you need to know about Zika.
To see the CDC list of sites where Zika virus is active and may pose a threat to pregnant women, click here.