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.”