Areas with the Zika virus had a higher rate of birth defects in the last half of 2016 than in the first half, but it is too soon to tell if the spike is caused by the virus, according to the Centers for Disease Control and Prevention.

The CDC found that areas with local transmission of the mosquito-borne virus — southern Florida, a part of southern Texas and Puerto Rico — saw a 21 percent increase in births with outcomes linked to Zika in the last half of 2016 from the first half of 2016. Zika is linked to birth defects including microcephaly, which causes babies to be born with abnormally small heads and brain damage.

“It is not known if this increase is due to local transmission of Zika virus alone, or if there are other contributing factors,” the CDC said. “Most of the mothers who had babies with the Zika virus-linked birth defects did not have laboratory evidence of Zika virus infection — either because they were not tested, were not tested at the right time, or were not exposed to Zika virus.”

The agency looked at 1 million births in 2016 in 15 states and territories. It found that about three out of every 1,000 babies born in the areas that year had a birth defect associated with Zika.

For instance, 49 percent of the babies born with defects had brain abnormalities or microcephaly. Another 22 percent had nervous system damage such as deafness or joint problems.

CDC hinted that more data from 2017 could show an increase in possible Zika-related birth defects. It said many pregnant women exposed to Zika in late 2016 gave birth in 2017.

The virus hit Puerto Rico hard. As of December, 4,690 pregnant women in U.S. territories had lab evidence of Zika, the majority of cases in Puerto Rico, the CDC found.

The rest of the U.S. had 2,364 pregnant women with Zika, CDC added.