Zika Update: Birth Defects Still Likely Even If Mother Has No Symptoms

Pregnant women who are infected with the Zika virus may give birth to babies with birth defects even if they don't exhibit the symptoms, report said.

The report, which was published in the New England Journal of Medicine, explored the possibility of babies being born with microcephaly, a condition where babies are born with abnormally small heads and brain development problems.

The report also details the Zika outbreak in Colombia where 65,726 people, including nearly 12,000 pregnant women, were reported to have Zika virus infections from August 2015 to April 2016.

Among cases of microcephaly reported in Colombia from January 1 to April 28 of this year, four infants were born with microcephaly and had been lab-confirmed to be infected by the virus.

However, The Washington Post reported that none of the mothers of the infected infants had exhibited Zika-related symptoms during pregnancy and were not reported to be part of the government's monitoring.

The most common Zika symptoms include fever, rash and joint pains. Only about 1 in 5 people with Zika infections show these symptoms.

"This is really adding weight to existing data that asymptomatic infection is also associated with microcephaly," Margaret Honein, chief of the birth defects branch at the Centers for Disease Control and Prevention (CDC) and one of the authors of the report, said in a statement published in The Washington Post.

The 12,000 pregnant women reported to have Zika are the only the ones who have symptoms, which means that there could be more infected pregnant women, those who don't have symptoms, Honein said.

Researchers also looked at a group of nearly 2,000 of these pregnant women. It was found that although 90 percent were infected during their third trimester, none of these infants had been born with birth defects.

While the Colombia data is still preliminary, Honein said that the report still gave "reassuring news about infection in the third trimester." But researchers still strongly advise mothers to continue monitoring their pregnancies and to do follow up on the infant outcomes.

Over half of all pregnancies in Colombia were unintended and less than half of sexually active women between the ages 15 and 24 are using condoms during sexual intercourse, although 61 percent of women used contraception.