Gloria Golú, 41, in her bed at at a hospital in Cali on June 30. She is expecting her third child and has been infected with the Zika virus. (Eduardo Leal/For The Washington Post)

In the nine months since the Zika virus appeared in Colombia, the government has reported nearly 100,000 cases, including more than 17,000 among pregnant women. But the epidemic has not produced the dreaded wave of fetal deformities witnessed in Brazil.

Brazilian health officials blame the virus for at least 1,600 cases of the birth defect microcephaly, and they are investigating another 3,000 for a link to Zika. In Colombia, the country that has logged the second-highest number of infections, authorities have linked Zika to 18 cases of microcephaly, with 112 under investigation.

The fact that Colombia has seen a relatively modest — not massive — increase in birth defects raises hopes that the threat of microcephaly from Zika is not as high as was feared when the images first surfaced of so many newborns in Brazil with small, misshapen heads.

“So far we haven’t seen an explosion of microcephaly in other countries [outside Brazil], and that is good news,” said Marcos Espinal, director of the communicable diseases and health analysis department at the Pan American Health Organization. “But we have to wait to see what happens.”

Researchers at the U.S. Centers for Disease Control and Prevention and other experts say there is more evidence than ever that Zika can interfere with fetal brain development. They caution that it is still too early to draw sweeping conclusions based on the preliminary Zika data from Colombia, particularly as American officials prepare for the possible spread of the virus this summer and an increase in U.S. cases.

But several countries where Zika is also thought to be widespread, including Haiti and Venezuela, have not reported any microcephaly. In El Salvador, where authorities urged women to avoid pregnancy for two years because of the virus, there have been only two cases.

Few other nations are tracking Zika with the same rigor as Colombia. While Brazil was essentially caught off guard by the outbreak, Colombia had a comprehensive monitoring effort in place by the time the first infections were confirmed last October.

Colombian health officials publish detailed weekly bulletins that report the number of infections and their location, as well as statistics on Zika-related complications including microcephaly and Guillain-Barré syndrome, a rare and sometimes fatal autoimmune disorder that appears to be triggered by the virus.

The CDC is working closely with Colombian doctors to identify Zika’s risks through “enhanced surveillance” programs in three cities — Cucuta, Barranquilla and Cali — where researchers will monitor fetal development in infected mothers and track the babies’ growth for at least a year after birth.

Cali and the surrounding department of Valle del Cauca have reported nearly 25,000 Zika infections, more than any other territory in Colombia. But as of last month there had not been a single case of microcephaly linked to the virus.

“I was so worried that something would be wrong with my baby,” said Gloria Golú, 41, who contracted Zika in week 16 of her pregnancy. But subsequent tests have detected no abnormalities, and at 30 weeks, her son appears healthy.

A visual journey through Recife

“I’m so happy that his head is normal,” she said.

Her obstetrician-gynecologist at the university public hospital in Cali, Gustavo Delvasto, said he and his colleagues braced for the worst when they saw the reports from Brazil and were told to prepare for a wave of Zika-induced birth defects. “Fortunately, it just hasn’t happened,” said Delvasto, who performs about 120 ultrasound exams per week.

Several conditions, including syphilis and fetal alcohol syndrome, can also cause microcephaly, and in an average year, Colombia has about 150 cases. This year the number will probably be between 200 and 300, according to Martha Lucía Ospina, the director of Colombia’s National Institute of Health.

That increase is linked to Zika, she said. “We’ve also received evidence of a large increase in miscarriages, of nearly 8 percent,” Ospina said.

Zika’s greatest threat to fetal brain development appears to be late in the first trimester of pregnancy. The Zika epidemic in Colombia peaked in February, so mothers who were infected then are due to give birth in the coming weeks, and Ospina said health officials expect an increase in microcephaly cases.

When World Health Organization official Bruce Aylward was asked last month why the microcephaly numbers in Colombia are so much lower than in Brazil, he told reporters that the country is only counting “live births” — indicating that many Colombian women with Zika are miscarrying or seeking to interrupt their pregnancies.

But Ospina said that doctors in Colombia who detect birth defects such as microcephaly through ultrasound exams are instructed to report those cases to health officials. Several providers of abortion services in Colombia, where the procedure is tightly restricted, said they had not seen a rise in the number of women seeking to interrupt pregnancy because of Zika fears.

One reason global health officials are reluctant to draw too many conclusions about the relatively low number of microcephaly cases outside Brazil is that they simply don’t know how many infections that country has experienced.

With a population of 200 million, Brazil is much larger than any other nation in South America, and unlike in mountainous Colombia, virtually its entire population resides in low-elevation areas that are home to mosquitoes that can transmit Zika.

The comparatively large number of microcephaly cases in Brazil may simply be an indication of how extensive the epidemic has been. Brazil didn’t require physicians to report suspected Zika cases until the outbreak was full blown.

Brazil’s northern state of Pernambuco, with a population of 9.2 million, has registered at least 366 cases of Zika-linked microcephaly since last year.

The WHO and others have sought to puncture Internet-fueled rumors that blame the surge of birth defects in northern Brazil on genetically modified mosquitoes or the use of pesticides.

Margaret Honein, head of the CDC’s birth defects branch, said that researchers are also looking at other potential causes for Brazil’s high number of microcephaly cases, including genetic factors or the simultaneous presence of additional infections, such as dengue fever, that could interact with Zika. “My anticipation is that timing in pregnancy will be the largest single factor,” Honein said, in an interview.

“If you have a higher percent of women infected during the period of highest risk, you’re likely to have more adverse outcomes,” she said.

In French Polynesia, where the same Asian strain of Zika that has reached the Americas produced an outbreak in 2013 and 2014 that infected more than 30,000 people, researchers linked the virus to eight cases of microcephaly.

In the United States, the CDC has linked Zika to 12 cases of suspected birth defects or miscarriages, out of 320 women who have laboratory evidence of the virus. Mosquitoes are not yet spreading Zika on the U.S. mainland, so all of those cases were acquired abroad or through intercourse with an infected partner.

Julia Symmes Cobb in Bogota contributed to this report.