By Susan Scutti

CNN

(CNN) -- The proportion of Zika-related birth defects during 2016 was nearly 20 times higher than the number seen during the pre-Zika years, according to a new report from the United States Centers for Disease Control and Prevention.

In particular, brain abnormalities or microcephaly, where a baby's head size is smaller than expected, occurred about 33 times as often in pregnancies with Zika infections than in the pre-Zika years.

Zika, a virus transmitted primarily by mosquitoes and unprotected sex, has been spreading around the globe with locally transmitted cases in the US were first reported in Florida and Texas during 2016. Currently, there is no vaccine to prevent infection.

Last year the World Health Organization declared a Public Health Emergency of International Concern because of the rapid pace with which the virus was spreading and the alarming number of cases of microcephaly seen in Brazil where large numbers of women had been infected. At the time the link between Zika and microcephaly was still uncertain but has since been proven.

"I think women should take that this further demonstrates how important it is to prevent Zika virus infection during pregnancy," said Peggy Honein, epidemiologist and chief of the birth defects branch at the CDC. "These devastating effects have a major increase over the baseline when Zika virus infection occurs during pregnancy."

Zika-related birth defects, including microcephaly, neural tube defects and eye defects, were seen in about three out of every 1,000 births during 2013-2014 compared to 60 out of every 1,000 completed pregnancies where the mother tested positive for a Zika infection.

Honein emphasized the number of women who have shown lab evidence of Zika virus infection as tracked by the CDC through a national registry.

"You'll see there's over 1,500 pregnant women in the 50 US states and D.C. with lab evidence of possible Zika virus infection and more than 50 fetuses and infants that have had one of these Zika-related birth defects," she said. "We are continuing to see pregnant women with Zika virus infection."

Rising numbers

In 2016, the CDC defined all the birth defects that are potentially related to a Zika virus infection. These include brain abnormalities such as microcephaly; neural tube defects and spina bifida; eye abnormalities, such as cataracts and optic nerve abnormalities; and consequences of central nervous system dysfunction, such as joint contractures that prevent normal movement.

Neural tube defects are "serious birth defects of the brain and spine and includes anencephaly (incomplete brain and/or skull) and spina bifida (spinal cord not completely closed)," said Michele Kling, a spokeswoman for the March of Dimes, in an email.

To assess the incidence of birth defects before and after Zika virus was introduced in the United States, the CDC researchers used data from surveillance programs in Massachusetts, North Carolina and Atlanta. For the new report, CDC researchers analyzed data from 2013-2014 and compared this with the US Zika Pregnancy Registry records from 2016.

For the early period, the researchers identified 747 infants and fetuses with one or more of these defects in the surveillance databases.

By comparison, data from the later period indicated 26 infants with these same birth defects among the 442 completed pregnancies of women with possible Zika infection.

Brain abnormalities or microcephaly were the most frequent conditions reported, followed by neural tube defects. More than half of those with brain abnormalities or microcephaly or with neural tube defects and other early brain malformations had evidence of these defects noted prenatally -- 55% and 89%, respectively.

Pregnancy losses (48%) and preterm delivery (66%) occurred most frequently with neural tube defects and other early brain malformations. Maternal age was similar across birth defect categories.

"Other data has shown us the highest risk is the first trimester and early in the second trimester but we don't think there's any safe time in pregnancy to be exposed to Zika virus infection," said Honein.

She said pregnant women should not travel to areas where Zika virus transmission and they should also avoid sexual contact with partners who have traveled to areas with Zika virus transmission.

According to Dr. Paul E. Jarris, the chief medical officer of the March of Dimes, the report of increased birth defects caused by Zika should be a wake-up call for anyone who thinks Zika is no longer a serious threat.

When families learn their child has a birth defect like those associated with Zika virus infection, they experience a grief reaction similar to when they lose a child, he said.

"There's also often guilt associated with it and they're overwhelmed by the complexity. So it has a pervasive effect on the family, including higher rates of divorce among couples" and financial stress, said Jarris. "It's devastating, that's why we have to really prevent this."