More than 250 pregnant women in the United States, worried they had been exposed to the Zika virus during their travels, have sought testing for the disease in recent months, the Centers for Disease Control and Prevention said Friday.

The overwhelming majority of the 257 women who requested lab tests – about 97 percent – turned out not to have the virus. But the CDC said it had been tracking nine pregnant women who did test positive for Zika, and their varied experiences highlight the angst and uncertainty accompanying the virus’s rapid spread, particularly concerns over severe birth defects associated with the illness.

CDC officials reported Friday that at least two pregnant women in the United States infected with the Zika virus chose to have abortions. Two others had miscarriages, though officials said they could not say what role Zika played. One woman gave birth to a infant with serious birth defects; two others delivered healthy infants. Two are still pregnant.

The women all reside in the United States but had traveled to at least one of dozens of countries and territories with active Zika infections.

Denise Jamieson, a CDC researcher helping oversee the agency’s response to the outbreak, said the number of brain abnormalities observed among the small group of pregnant women with Zika was higher than experts would have expected.

One of the U.S. women who chose to have an abortion was in her 30s and had contracted the virus while traveling during her first trimester, the CDC reported. When she was 20 weeks pregnant, an ultrasound showed that her fetus had severe brain abnormalities. Doctors also found the Zika virus in her amniotic fluid.

Symptoms prevalent

The CDC said the nine pregnant women who tested positive for Zika all had reported symptoms of the disease, such as fever, rash and joint pain – a notable fact, given that the majority of people infected by the virus do not display symptoms.

Six of the pregnant women were infected with the virus during their first trimester, the CDC said. In addition to the two elective abortions and two miscarriages, one of the woman delivered a baby who suffered from “severe microcephaly,” as well as seizures, trouble swallowing, eye problems and calcifications in the brain. The other woman is still pregnant.

Two women were diagnosed with Zika during their second trimester, the CDC said. One gave birth to an apparently healthy baby; another is still pregnant. A pregnant woman who experienced Zika symptoms during her third trimester delivered a healthy infant.

Friday’s report adds to the ongoing efforts by scientists in the United States and abroad to decipher the many mysteries surrounding the once-obscure Zika virus. At the top of that list is determining whether – and how – the virus is linked to birth defects such as microcephaly, as well as to cases of a rare autoimmune disorder known as Guillain-Barré syndrome.

CDC Director Tom Frieden said Friday that scientists are increasingly confident of the association between the Zika virus and microcephaly. But much remains that they still do not understand, such as whether the risks are higher at certain points during pregnancy.

“There are many things we wish we knew and are working hard to find out,” Frieden told reporters Friday. “This is an extraordinarily unusual occurrence. . . . Part of science is uncovering information step by step, trying to be sure not to overstate what the data shows.”

Zika has confounded researchers at every turn. Earlier this week, the CDC said it was investigating 14 new reports of potential sexual transmission of the virus, suggesting that that method of infection – rather than just mosquito bites – is more likely than scientists had realized. “We did not anticipate that we’d see this many sexually transmitted cases of Zika,” Frieden said.

Richard Beigi, chief medical officer at the University of Pittsburgh Medical Center’s Magee-Womens Hospital, said Friday’s CDC report reinforces fears that many pregnant woman already have about the virus but does not provide clear answers.

For example, the CDC recommends increased surveillance starting at 18 to 20 weeks of pregnancy for women suspected of having a Zika infection, but there is little a woman can do if signs of microcephaly begin to surface. “The big challenge right now is we don’t have any interventions,” Beigi said.

There is no vaccine and no treatment for Zika or the possible health problems associated with it. Doctors also cannot give women infected with Zika a clear sense of the prognosis for their infants. “It’s too early to know what this is going to mean for the babies after they are born and when they grow up,” he said.

Unlike some chromosomal issues that can be detected in the womb with a very high degree of certainty, a child’s long-term trajectory can be less clear-cut with microcephaly. Ultrasounds can show whether a child’s head is abnormally small as early as in the second trimester. But while head size sometimes correlates with the severity of the impact, that is not always the case. Experts estimate that as many as 10 to 15 percent of people diagnosed with the condition at birth do not have any intellectual impairment or other issues.

“Even if certain parts are damaged or underdeveloped,” said Ganeshwaran Mochida, a neurologist at Boston Children’s Hospital, “sometimes with enough intervention early on, we are surprised how much they are capable of.”

Such ambiguities only complicate the already difficult predicament pregnant women face when they have been infected with Zika, which has raced through much of the Americas in recent months.

Throughout Latin America, home to some of the most restrictive abortion laws in the world, there have been controversial calls to lighten restrictions on the practice in the face of the Zika outbreak.

Pope Francis himself has waded into the issue, saying recently that the use of artificial contraceptives may be morally acceptable in combating the Zika virus, even as he condemned abortion as an option. “Abortion isn’t a lesser evil – it’s a crime,” he said this month. “Taking one life to save another, that’s what the Mafia does. It’s a crime. It’s an absolute evil.”

Frieden said Friday that the United States has 147 reported Zika cases in 24 states and the District of Columbia, most related to people who traveled to areas affected by the virus.