At least four published studies have cited cases in which women with no memory of having symptoms gave birth to children with birth defects.
The new case definition includes all women who have had a positive test for Zika virus, or are likely to test positive for antibodies to it.
“C.D.C. wants to make sure we are monitoring all women at risk for poor outcomes,” said Dr. Denise J. Jamieson, a co-leader of the pregnancy and birth defects team at the agency.
Each Thursday, the agency will update the number of infected pregnant women in the United States, officials said.
The outbreak in Cape Verde, a Portuguese-speaking country with close links to both Brazil and West Africa, has been a puzzle for months.
The intense outbreak in the island nation started last October, just as Brazil’s surge in microcephalic babies was detected. But confirmation that it was caused by the Brazilian strain, rather than the much older African one, did not come until the Pasteur Institute in Dakar, Senegal, sequenced the genomes of the infecting viruses.
Cape Verde’s outbreak is now fading, said Dr. Matshidiso Moeti, the W.H.O.’s regional director for Africa, and there has been no outbreak in Senegal, which she called Cape Verde’s “door out to other countries.”
Although that might suggest that Africans have some immunity, “it’s never good news to hear that a virus with neuropathic effects is on your doorstep,” said Dr. Bruce Aylward, the W.H.O.’s chief of emergency response.
“We don’t know what the level of population immunity is in Africa, and we don’t know if it will mitigate some of the impact,” he said. “So we have to work against a worst-case scenario.”
Although laboratory capacity in Africa has been built up in the wake of the 2014 Ebola outbreak, the continent is still very poor and the response to the W.H.O.’s appeal to donors for a $50 million Zika fund has been “anemic,” Dr. Aylward said.
A recent study in Clinical Microbiology Reviews looked at 80 sets of blood tests for the Zika virus in countries around the world between 1947, when the virus was discovered in Uganda, and 2014, when it was detected in French Polynesia — the last major outbreak before it reached South America.
Some samples are small, some tests used methods that would confuse Zika with related viruses, and the results range wildly: For example, 12 percent of 261 Ugandans tested in the 1940s were immune to the virus, while nearly 60 percent of 2,457 Senegalese tested in the 1970s were.
The authors concluded that the Zika virus was probably endemic to all of sub-Saharan Africa, although there was no way to know how many people had immunity at any given time.
Although Dr. Aylward said it is still not known whether immunity to the African strain confers protection from the Brazilian one, Scott C. Weaver, the director of the Institute for Human Infections and Immunity at the University of Texas Medical Branch in Galveston and an expert on the Zika virus, said he would be “very surprised if there weren’t complete cross-protection between the two.”
Genetically, the two strains are very closely related, he said. By contrast, strains of yellow fever have much more variation, and yet he did not know of a single instance in which the yellow fever vaccine, which is based on one strain gathered years ago, failed to protect against the other strains.
Testing in animal models, including immune-deficient mice, has just begun, he said, but he could not name any laboratory trying to prove that infection with one strain of Zika protects against later infection with another.
“We assume there’s cross-protection, and we’re interested in other questions,” he said, including whether microcephaly is typically caused by Zika alone.
At the White House on Friday, President Obama pressured Congress to give him $1.9 billion for the fight against Zika.
Americans should tell their Congressional representatives to “get on the job,” he said.
“This is something that is solvable — it is not something that we have to panic about, but it is something we have to take seriously,” Mr. Obama said in the Oval Office after a briefing from top health officials.
Congress should not take its scheduled Memorial Day vacation without passing the legislation, he said.
His advisers have said the president would veto a House measure that provides only $622 million. The administration was “encouraged” by the Senate’s $1.1 billion version but is not promising to sign it.
Referring to the funding Mr. Obama has requested, “anything short of that will be insufficient,” said Eric Schultz, the deputy White House press secretary.