The findings are based on the US Zika Pregnancy Registry, which was established last year to track Zika among pregnant women in the United States and its territories (except Puerto Rico, which has a separate registry).
included all 972 completed pregnancies from January through December 2016. Birth defects were reported in 5% (or 51) of the pregnancies with laboratory evidence of possible Zika infection, including six pregnancy losses that may have been due to miscarriage, stillbirth or termination.
That number climbs to 10% when looking at women with confirmed Zika infections.
Birth defects were reported in 15% of women who became infected with the virus during the first three months of pregnancy, known as the first trimester. This is in line with reports indicating that the infection during the earlier part of pregnancy poses a greater risk to the fetus.
Reported birth defects include microcephaly, in which the brain does not develop properly, and other brain abnormalities.
"We are still learning about the range of birth defects. ... (We've) seen it can cause vision, hearing, brain problems and others," said Dr. Anne Schuchat, the CDC's acting director. She added that babies can have seizures, restricted joint movement, feeding difficulties, trouble sitting up and inconsolable crying.
What's worse, the effects of the virus aren't always obvious at birth and may develop later, which is why screening is critical for all babies born to women who may have been infected while pregnant. That screening includes brain imaging with an ultrasound or a CT scan, as recommended by the CDC.
Yet the new report found that only 24% of children of women who might have been infected during pregnancy were given brain scans after they were born.
"Because we don't have brain imaging results for most babies (captured by the registry), our numbers may be low," said Peggy Honein, chief of the CDC's birth defects branch. "Identifying these babies as soon as possible after birth is important to ensure they receive the best care possible." That care is estimated to cost up to $10 million in infants who survive into adulthood.
Last month, the CDC reported that the proportion of Zika-related birth defects last year was nearly 20 times higher than what was seen during the pre-Zika years.
Although the Zika pregnancy registry includes all 50 states, pregnancies in this new report were reported in only 44 states. All 51 birth defects reported Tuesday were cases in which the mothers were infected while traveling to 16 areas where the virus is circulating: Belize, Barbados, Brazil, Cape Verde, Colombia, the Dominican Republic, El Salvador, Guatemala, Guyana, Haiti, Honduras, Jamaica, Mexico, Puerto Rico, the Marshall Islands and Venezuela.
Schuchat noted that where a person becomes infected has no impact on Zika-related pregnancy complications.
The CDC reminds women who are pregnant or trying to become pregnant to avoid travel to areas where the virus is circulating and to follow recommendations for preventing sexual transmission of the virus if their partner has traveled to such an area.
Every mosquito bite carries a risk, Schuchat said, and therefore preventing mosquito bites is critical to keep pregnant women and their babies safe. "Don't let this outbreak become your family's heartbreak," she said.
More than 5,100 cases of the virus have been reported in the continental US and Hawaii. Only a couple hundred cases of local transmission have been reported, as most people are infected while traveling to areas where Zika is circulating.
Schuchat noted that 30 to 40 pregnant women are reported to the Zika registry each week. As of March 14, the most recent data
available, the registry counts 1,617 women in the United States and its territories other than Puerto Rico.
The CDC counts
1,228 completed pregnancies without known
birth defects, 54 babies with Zika-related birth defects and seven pregnancy-related losses
since the registry began last year. There are 389 women currently pregnant on the registry.