RECIFE, Brazil — The surging medical reports of babies being born with unusually small heads during the Zika epidemic in Brazil are igniting a fierce debate over the country’s abortion laws, which make the procedure illegal under most circumstances.

Prominent legal scholars in Brasília, the capital, are preparing a case to go before Brazil’s highest court, arguing that pregnant women should be permitted to have abortions when their fetuses are found to have abnormally small heads, a condition known as microcephaly that Brazilian researchers say is linked to the virus.

A judge in central Brazil has taken the rare step of publicly proclaiming that he will allow women to have legal abortions in cases of microcephaly, preparing the way for a fight over the issue in parts of the country’s labyrinthine legal system.

And here in Recife, the Brazilian city hit hardest by the increase in microcephaly and the brain damage that often comes with it, abortion rights activists are seizing on the crisis to counter conservative lawmakers who have long wanted to make Brazil’s abortion laws — already among the most stringent in Latin America — more restrictive.

The scientific link between Zika and infant brain damage has not yet been proven. But the rising reports of microcephaly in parts of Brazil stricken by Zika have caused enough alarm that the World Health Organization declared an international public health emergency on Monday, noting that its “experts agreed that a causal relationship between Zika infection during pregnancy and microcephaly is strongly suspected.”

Some Brazilian doctors are already encountering pregnant women seeking abortions because of the spike in microcephaly cases. Dr. Artur Timerman, an infectious disease specialist in São Paulo, said that two patients had spoken with him in recent weeks about ending their pregnancies because they tested positive for the Zika virus.

“They come to my office and ask, ‘Is there a chance for my baby to have microcephaly?’” Dr. Timerman said. “We need to inform them there is. They ask if the chance is big or small. I respond, ‘I don’t know.’ They ask what I would do in their position. I tell them it’s a personal decision, only that the chance is a real one.”

“Later,” Dr. Timerman said, “both patients told me they had abortions.”

The debate over whether women should be allowed to have abortions in microcephaly cases could reverberate across the region. The outbreak in the Western Hemisphere is believed to have begun in Brazil, the country with the most Zika infections by far. But the epidemic has spread to more than 25 countries and territories in the Americas, some of which have abortion laws that are as restrictive as Brazil’s, if not more so.

The push to relax abortion restrictions in Brazil raises difficult issues on many sides of the argument. The most severe cases of microcephaly can usually be detected with ultrasound scans around the end of the second trimester, or roughly 24 weeks. Supporters of Brazil’s existing abortion laws contend that such late-term abortions intensify an already wrenching decision.

“With microcephaly, the child is already very much formed and the parents are conscious of this,” said Dr. Lenise Garcia, a biology professor at the University of Brasília and the president of Brazil Without Abortion, an organization against easing the abortion laws. “Getting an abortion creates guilt that will stay with the woman for the rest of her life.”

Judge Jesseir Coelho de Alcântara, who has publicly stated that abortion should be allowed in microcephaly cases, acknowledged that the issue is complex.

“I know this is very difficult because the subject is new, requires thorough discussion and a great deal of religious influences persists,” said Mr. Coelho de Alcântara, a judge in Goiás State. “But my position is that abortion for microcephaly should be allowed.”

Proponents of changing the abortion law cite a 2012 ruling by the Supreme Federal Tribunal of Brazil allowing abortions when the fetus has anencephaly, a serious birth defect in which parts of the brain or skull are missing. Almost all babies with anencephaly die shortly after birth, the Centers for Disease Control and Prevention says.

But microcephaly is far less predictable. Even when it is detected before birth, doctors often cannot say what the effects will be, potentially making decisions about abortion a lot more complicated.

“Some children with severe-appearing brain malformations seem to be relatively unaffected,” said Dr. Hannah M. Tully, a neurologist at Seattle Children’s Hospital who specializes in brain malformations. “Yet others with relatively minor structural problems may have profound disabilities.”

At least 10 percent of babies with microcephaly have no mental deficits. Indeed, these children end up “intellectually and developmentally normal,” said Dr. Constantine A. Stratakis, a pediatric geneticist and a scientific director at the National Institute of Child Health and Human Development in Bethesda, Md. But any child whose head measures “three or four standard deviations below the mean, then it’s very unlikely that you will be dealing with normal intelligence.”

In Brazil, abortions are allowed only in cases of rape, anencephaly or when the mother’s life is in danger. Until recently, conservative lawmakers had been seeking to make legal abortions harder to get, reflecting the influence of Roman Catholic leaders and the increasingly powerful preachers at the helm of a growing evangelical Christian movement.

Led by Eduardo Cunha, the conservative speaker of Brazil’s lower house, an influential bloc of evangelical Christian lawmakers introduced legislation in 2015 to make it harder for rape victims to get abortions by requiring them to undergo a police report and forensic medical exam. Another part of the bill seeks to make it a crime for people to assist in an abortion or encourage a pregnant woman to have one.

“Pregnant women across Brazil are now in a panic,” said Silvia Camurça, a director of SOS Corpo, a feminist group in Recife. “The fears over the Zika virus are giving us a rare opening to challenge the religious fundamentalists who put the lives of thousands of women at risk in Brazil each year to maintain laws belonging in the dark ages.”

As in the United States before the Supreme Court’s legalization of abortion in 1973, a clandestine abortion industry thrives throughout Brazil. Some illegal providers charge thousands of dollars to do the procedures, risking arrest and the closing of their clinics.

Estimates on the number of illegal abortions in Brazil vary widely. Drawing on hospital records showing that about 150,000 women seek medical attention each year for complications from illegal abortions, Brazilian scholars estimate that as many as 850,000 abortions are performed illegally in the country on an annual basis.

While Brazil’s abortion laws are less stringent than those in some other Latin American countries — in El Salvador, for instance, abortion is not allowed under any circumstances — illegal procedures are not treated lightly.

One Brazilian woman was handcuffed to a hospital bed and arrested after she sought medical attention for a botched abortion. A judge sentenced other women in the city of Campo Grande who had undergone illegal abortions to do community service in day care centers, arguing that it would teach them to love children. A 9-year-old girl who said she had been raped by her stepfather was allowed to have an abortion in Recife, but only after a heated national battle in which officials overcame objections from religious leaders.

Debora Diniz, an anthropologist and researcher at Anis, an abortion rights group planning to file a lawsuit seeking to legalize abortion in cases of microcephaly, likened the Zika crisis to the long struggle to allow abortion in cases of anencephaly, which lasted about a decade.

“We have an epidemic, an emergency, and the public health sector is not properly caring for women’s rights,” she said. “We have constitutional rights at risk, the right to health care and human dignity.”

Religious leaders are vowing to resist any effort to ease Brazil’s abortion laws because of Zika.

“Nothing justifies an abortion,” the Rev. Luciano Brito, a spokesman for the Roman Catholic Archdiocese of Olinda and Recife, told reporters. “Just because a fetus has microcephaly won’t make us favorable” to changing the law.