Officials have not said which states they are working with or where these women live because the risk applies to all women in the United States, according to Dr. Jennifer McQuiston, deputy incident manager for Zika virus at the CDC.
The CDC's already advised travelers to be aware of the risk, recommending that men who have traveled to Zika-affected zones should use a condom if they want to be absolutely sure they don't infect sex partners.
There's still a whole lot of uncertainty over how sexual transmission would work, and the CDC is now researching important questions, such as how long Zika can stay present in semen, when men are the most infectious, and which sex acts can pass the virus on.
Meanwhile, CDC also published a new guidance to avoid getting infected with Zika.
Although all cases of sexual transmission so far have come from male partners, interestingly, the CDC added, "At this time, there is no evidence that women can transmit Zika virus to their sex partners; however, more research is needed to understand this issue".
Zika is now spreading through Central and South America and the Caribbean, and with the high volume of news about the virus, it's tough to stay up-to-date.
All of the newly-reported cases of sexual transmission have been in the US.
Although sexual transmission of Zika virus infection is possible, mosquito bites remain the primary way that Zika virus is transmitted. They hope to recruit at least 130 babies with microcephaly and compare them to nearly triple that number of infants without the condition, the AP said. Treatment is symptomatic since there is no specific treatment against the virus. That same mosquito is found in the Southern U.S. and officials expect they will eventually spread the virus, too.
Several of the cases involve partners who are pregnant. The mosquito transmits the Zika virus and is being studied at the institute.
US and Brazilian health workers seeking to determine if the Zika virus is causing a surge in birth defects ran into the chaotic reality of northeastern Brazil on their study's first day Tuesday.
The projections reflect the World Bank's expectation that the Zika virus will inevitably lead to a drop in tourism arrivals in the region.
Meanwhile, the Florida Department of Health said 29 Zika cases were reported in the state - the newest case in Seminole County.
It also includes a graphic meant for pregnant women considering travel to Zika-affected areas. The idea is to determine whether mothers whose babies have microcephaly were infected with Zika and, if so, when during their pregnancies.
Evidence of Zika's risk to pregnant women continues to grow
This year's case comes as the state fights an outbreak of mosquito-borne dengue fever, with 260 cases reported on the Big Island. Researchers at Johns Hopkins said they have found a new biological link between the Zika virus and microcephaly in babies.