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Why the number of pregnant US women with possible Zika infections just tripled

If you've been watching cable news, you might get the impression that an epidemic of Zika among pregnant women has suddenly spiraled out of control in the United States.

According to a report on Friday from the Centers for Disease Control and Prevention, 279 pregnant women with possible Zika infection in US states and territories are being monitored — tripling the agency's previous estimates.

Have cases suddenly spiked? Not exactly. What actually happened is that the CDC changed the method it's been using to count cases. The new approach uses a broader definition — and one that better reflects what we're learning about how the disease can affect fetuses.

Until today, the CDC only looked at women who had both evidence of Zika virus in blood tests as well as symptoms or complications during pregnancy. That methodology was based on the belief that there was an association between symptomatic women and birth defects associated with the virus, like microcephaly — which is characterized by a shrunken head and incomplete brain development. (Zika symptoms include fever, rash, joint pain, and red eyes — but they only occur in a minority of people who have the virus.)

Newer research has indicated that some women who test positive for the virus but recalled having no symptoms later delivered babies with microcephaly and other brain defects.

By the new count, there were 157 pregnant women on the US mainland with possible Zika infection and 122 pregnant women in Puerto Rico — a total of 279 women. The CDC had previously said it was monitoring just 113 pregnant women, a number that excluded those who didn't report Zika symptoms or pregnancy complications.

The CDC has not yet reported on the outcomes of pregnancies they're monitoring.

Zika definitely causes birth defects

According to the Centers for Disease Control and Prevention, there's indisputable evidence that Zika causes microcephaly.

The association between the condition and Zika has been seen in this outbreak in Brazil, as well as in reanalyses of the data from previous outbreaks, such as one in the French Polynesian islands in 2013 to 2014. Other countries, including Cabo Verde, Colombia, Martinique, and Panama, have also reported microcephaly or other fetal malformations to the WHO over the last year.

In addition to microcephaly, researchers have found that the virus seems to kill off the tissue in entire regions of the brain, damage babies' eyes, and heighten the risk of miscarriage and fetal death.

Researchers believe pregnant women are at a greatest risk of having babies with birth defects if they are infected in the first trimester. They've also estimated that women infected with Zika during the first trimester of their pregnancies face a one in 100 chance of delivering a baby with microcephaly.

Zika wouldn't be the first virus to cause microcephaly. Rubella famously caused an epidemic of birth defects before the advent of the vaccine, and cytomegalovirus (CMV) can damage fetuses too.

But Zika is the first mosquito-borne virus to threaten fetuses. And even if birth defects turn out to be a very rare complication of Zika, the risk to fetuses was enough to prompt the WHO to declare a rare global public health emergency earlier this year.

Health officials urge pregnant women to avoid travel to places where Zika is circulating

In total, there have been about 550 cases of Zika in the United States — but most have originated in places where the virus is currently circulating, and a minority have occurred because of sexual contact with a traveler.

"Although the mosquitoes that carry Zika are here, Zika is not currently here," Dr. Denise Jamieson, an obstetrician-gynecologist and the chief of CDC's women's health and fertility branch, told us in April.

So she advised pregnant women in the continental US to avoid travel to areas where there is ongoing Zika transmission. (This includes more than 40 countries around the world.)

"For women who live here and are pregnant — they should avoid mosquito bites."