(CNN) The Centers for Disease Control is considering new guidelines for pregnant women traveling to areas where the Zika virus is present.

"We're trying to get out some comprehensive travel guidelines that women can follow, with more specific information for pregnant women, " said said Dr. Lyle Petersen, director of the Vector-Born Disease division of the National Center for Emerging and Zoonotic Infectious Diseases at the CDC. "We expect to have them out in a day or so."

The CDC was prompted by tests that found Zika, a mosquito-borne illness, in fetal and newborn tissue of Brazilian babies affected with microcephaly.

Microcephaly is a neurological disorder that results in babies being born with abnormally small heads, causing severe developmental issues and sometimes death.

"We now have an accumulating number of cases in babies from miscarriage or who were born with microcephaly with evidence of Zika," said Petersen.

"That suggests a stronger and stronger relationship of Zika and microcephaly."

The CDC often issues travel health notices. For example, last month they alerted travelers to Central America, Brazil, Mexico and Puerto Rico that Zika virus cases were occurring and precautions should be taken to prevent mosquito bites to reduce risk of infection. Similarly, last year they issued an advisory for travelers to the West African nations experiencing a deadly Ebola outbreak.

The World Health Organization and the Pan American Health Organization already have alerts in place for travel to Brazil and other countries in Latin America due to the current Zika outbreak.

Petersen said people often don't heed the warnings to protect themselves against mosquitoes while traveling. "Pregnant women need to take the best precautions possible when traveling where this [Zika] virus is spreading," Petersen said.

The Zika virus is part of the flavivirus family, which contains the deadly yellow fever virus, as well as West Nile, chikungunya, and dengue. There is no vaccine to prevent it and no medicine to treat a Zika virus infection. It's transmitted when a mosquito bites an infected person, and then spreads the obtained virus by biting others. In theory, the Zika virus could be spread through blood transfusion, but as yet there are no documented cases, the CDC said. There is one case of possible virus transmission via sexual contact.

'Pandemic in progress'

In November, Brazilian health officials advised women there not to become pregnant after they linked an increase in microcephaly to the Zika virus outbreak there. More than 2,400 suspected cases of microcephaly were found in 20 Brazilian states in 2015, compared with 147 cases in 2014. That number, said Petersen, has now risen to 3,700. Doctors there are investigating 29 related infant deaths.

"That's a pandemic in progress," said Dr. Anthony Fauci, director of The National Institute of Allergy and Infectious Diseases at NIH, who wrote a recent Zika-related editorial in the New England Journal of Medicine . "It isn't as if it's turning around and dying out, it's getting worse and worse as the days go by."

"And it's determined by officials in Brazil that peak mosquito transmitting time is in April, so we haven't begun to see the full impact of this outbreak," said Fauci.

Infectious disease control will need to step up its game if that reality comes to pass, said Fauci. "We don't have vaccines or therapeutics, and we need better diagnostics that can be done at point of care. We're aggressively working on a Zika vaccine but the best thing right now is avoidance or mosquito control."

In utero transmission

Petersen's CDC team tested human tissue from four babies born to Brazilian mothers who had all experienced a Zika-like fever and rash during early pregnancy. Zika was found in the brain tissue of two babies who were born full term with microcephaly, but died within 24 hours.

"The other two were miscarriages," said Petersen. "In both we found evidence of Zika in the placenta. In addition, Brazil has looked at a number of babies as well and found Zika in the amniotic fluid of two women whose fetuses had evidence of microcephaly via sonogram."

"What we now know," said Petersen, "is that fetuses can be infected with the virus. That's not new for infectious diseases, but it is new for this virus."

"This is a very remarkable and unusual situation," agrees Fauci, "because the other flaviviruses don't do that to our knowledge. You just don't see that with dengue or West Nile or chikungunya."

So far the biggest outbreak of microcephaly has been contained to Brazil. "There have been a small number of cases of microcephaly connected to a previous outbreak of Zika in the Polynesian Islands," said Fauci, "but that's still soft data and we haven't had the opportunity to firm that in yet. So we really can't say with any certainty whether it's a Brazilian strain or not."

Impact on the U.S.

Earlier this week health officials in Harris County, Texas, confirmed a case of Zika virus in a person who recently returned from Latin America, where the virus is endemic.

According to CDC spokesman,Tom Skinner, there have been 14 imported cases of Zika virus among returning U.S. travelers from 2007 to 2014. At least another eight imported cases were confirmed by CDC in 2015 and 2016, and they are still running tests on specimens from returning U.S. travelers who became ill last year and this year, so that number could rise.

The concern, of course, is whether these imported cases into the U.S. could result in locally transmitted cases. The Aedes mosquito, which transmits Zika virus, is present in many areas of the U.S.

"However, recent chikungunya and dengue outbreaks in the United States suggest that Zika outbreaks in the U.S. mainland maybe likely limited in size," Petersen said. Given this, it is important that we maintain and improve our ability to identify and test for Zika and other mosquito-borne diseases."

Other mosquito-borne diseases such as malaria and dengue fever, used to be widespread in the U.S. but screens on windows and doors, air-conditioning, and deliberate mosquito control efforts have helped to eliminate them, according to Petersen.

"Don't forget that we have an advantage in that most parts of our country go through significant winter," said Fauci. "That kills the mosquitos right off. And we handle local outbreaks in warm places like Florida with quick containment and eradication."

"So my message is don't worry right now at all about Zika being a problem in the U.S."

But when visiting other countries, "check with CDC about travel warnings and follow them to the letter."