Federal health officials have made a new color-coded map for Miami and say pregnant women should stay out of the "red zone" — where Zika virus is actively infecting new people daily.
And they say pregnant women should consider postposing all travel to Miami-Dade county for the time being, designating the entire county a "yellow zone."
"Pregnant women should specifically avoid travel to red areas because the intensity of Zika virus transmission confirmed in these areas is a significant risk to pregnant women," the CDC said in a statement.
Related: New Zika Zone Found in Miami
Last week, Florida health officials declared a new Zika zone — a one square mile area where the Zika virus is actively infecting people — in Miami. It's the third zone of active transmission, although home-grown cases have been found in several parts of the state.
Florida remains the only U.S. state with verified local transmission of the virus.
Florida has 174 locally transmitted cases, including 19 in out-of-state visitors. The state has 1,044 total cases of Zika, most related to travel. And 110 of those infected are pregnant women.
Zika normally causes mild illness at worst. But it's bad news if pregnant women get it.
There's no doubt among experts that Zika causes microcephaly, other types of brain damage and other birth defects. The virus has been shown to go directly to developing brain cells; it gets through the placenta and into a developing fetus. It's been found in the brain and other organs of babies with microcephaly and other brain damage.
"Zika continues to pose a threat to pregnant women living in or traveling to Miami-Dade County," said the CDC's Dr. Lyle Petersen. "Our guidance today strengthens our travel advice and testing recommendations for pregnant women, to further prevent the spread of the infection among those most vulnerable."
The CDC also says any pregnant woman with a connection to the area should get tested for Zika, even if she doesn't have symptoms.
"Pregnant women who have lived in, traveled to, or had unprotected sex with someone who lived in or traveled to Miami-Dade County since August 1, 2016, should be tested for Zika virus," the CDC advises.
"Pregnant women who have lived in, traveled to, or had unprotected sex with someone who lived in or traveled to the 4.5-square-mile area of Miami Beach with active spread of Zika virus since July 14, 2016, should be tested for Zika virus."
The CDC will use the new map going forward.
A red area is "a geographic area where local, state, and CDC officials have determined that the intensity of Zika virus transmission presents a significant risk to pregnant women," the CDC said.
"The intensity of Zika virus transmission is determined by several factors, including geographic distribution of cases, number of cases identified, known or suspected links between cases and population density."
A yellow area has had local transmission, but not intense transmission. "Although the specific level of risk in yellow areas is unknown, pregnant women are still considered to be at risk," the CDC said.
"Also, areas adjacent or close to red areas may have a greater likelihood of active spread of Zika virus and are considered to pose a risk to pregnant women."
The Aedes mosquitoes that transmit Zika do not themselves travel far, but infected people do, and they can pass the virus to mosquitoes in new areas.
"Currently, a 4.5-square-mile area of Miami Beach and one-square-mile area in Little River located in Miami-Dade County are red areas. The rest of Miami-Dade County is a yellow area," the CDC said.
Related: Questions About Pregnancy and Zika
CDC also has detailed guidelines for women planning to get pregnant.
"Given the limited available information about how long Zika virus can stay in body fluids and the chances of harm to a pregnancy when a woman is infected with Zika virus around the time of conception, some couples in which one or both partners have had a possible Zika virus exposure may choose to wait longer or shorter than the recommended period to conceive, depending on individual circumstances like age, fertility, and the details of possible exposure, and their risk tolerance," the CDC said.