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Why isn’t Zika causing birth defects in Colombia? It may only be a matter of time.

A 6-week-old baby born with microcephaly is treated at the Lessa de Andrade polyclinic during a physical therapy session in Recife, Brazil. In Colombia, over 2,000 pregnant women have been diagnosed with Zika, and authorities are watching to see if their babies have microcephaly. (Mario Tama/Getty Images)

BOGOTA -- No country outside Brazil has confirmed more Zika cases than Colombia — including more than 2,100 pregnant women who have had the virus.

But so far health officials have not found a single case of the birth defect known as microcephaly among those expectant mothers.

Is that a reason to doubt what the World Health Organization (WHO) said Monday was a "strong" suspicion that the mosquito-borne virus is to blame for Brazil's big increase in the number of babies born with unusually small heads and damaged brains?

In a word: no. It's just too early to tell, experts say.

Brazil and French Polynesia are the only two places so far where health officials have linked the virus to an apparent increase in microcephaly. This is one reason global health officials are watching Colombia closely for similar evidence.

“Colombia will tell us a lot,” said Marcos Espinal, director of communicable diseases and health analysis for the Pan American Health Organization.

Authorities there are monitoring pregnant women who have already been diagnosed with Zika, he said.

But one challenge is that the birth defects don't typically appear in a fetus until the end of the second trimester or the beginning of the third, according to experts. Other types of damage can't be detected until birth, or even later, they say — and many of the Zika cases are fresh. Researchers only detected transmission of Zika in Colombia last October, while it had emerged in Brazil at least five months earlier.

Cynthia Moore, an expert on birth defects at the U.S. Centers for Disease Control and Prevention, said it is very difficult to accurately determine microcephaly from the mid-pregnancy ultrasound that most women get as part of routine prenatal care at about 20 weeks.

“From reports by Brazilian physicians, it appears that around 30 weeks is when the microcephaly is best detected along with the ‎abnormalities of the developing brain,” she said.

Brazilian authorities have received more than 4,000 reports of suspected microcephaly cases linked to Zika. But in the first 762 cases they have examined, only 270 had microcephaly that appeared to be caused by Zika or another infection. (Microcephaly can also be caused by genetic disorders and such things as maternal alcohol abuse).

The WHO says it also suspects Zika could be linked to a neurological affliction called Guillain-Barre syndrome, which can cause paralysis.

Brazilian officials say that 400,000 to 1.5 million citizens may have been infected with Zika, and global health officials are bracing for 4 million cases in the Americas this year.

To date Colombia has reported more than 20,000 cases of the virus, which has spread "explosively" to 24 countries and territories in the hemisphere, according to WHO officials.

Zika has no vaccine or treatment, and researchers are only beginning to study whether it is definitively linked to microcephaly and other birth defects.

The city of Barranquilla, on Colombia's hard-hit Caribbean coast, has started fumigating the streets ahead of its annual Carnival celebrations. Almost 2,000 cases have been recorded in the area.

“We want communities to identify focal points of mosquitoes and report them to local authorities so they can be eliminated,” Colombian Health Minister Alejandro Gaviria said in a statement. “That way we stop the proliferation and the contagion.”

Gaviria has asked women to delay pregnancy for six to eight months and said confirmation of Zika infection — and possible damage to the fetus — could allow women to qualify for abortions, which are illegal except for circumstances such as rape or risks to the mother's health.

Nearly 40 percent of infected pregnant women in Colombia live in eastern Norte de Santander province, the national health institute said in a bulletin last weekend, along the country’s border with Venezuela.

The Health Ministry expects up to 650,000 Zika infections.

Read more:

Zika virus and microcephaly: scientists investigate whether mosquito bite is responsible for brain defect

Brazilians panic as mosquito-born virus is linked to brain damage in thousands of babies

As Zika virus spreads, El Salvador asks women not to get pregnant until 2018