The Zika virus can make thousands of copies of itself in fetuses' brains and in the placentas of pregnant women, which may help explain how the virus causes devastating birth defects and pregnancy losses even if a woman had only a minor illness, the U.S. Centers for Disease Control and Prevention (CDC) said Tuesday.

"Our findings show that Zika virus can continue to replicate in infants' brains even after birth, and that the virus can persist in placentas for months -- much longer than we expected," Julu Bhatnagar, lead of the molecular pathology team at CDC's Infectious Diseases Pathology Branch and the study's lead author, said in a statement.

The CDC researchers tested tissues from 52 patients with suspected Zika virus infection, including brain tissues from eight infants who had microcephaly and later died.

They also tested placental tissues from 44 women: 22 who had adverse pregnancy or birth outcomes, including miscarriage, elective termination, stillbirth or babies born with microcephaly, and 22 who had babies who appeared healthy.

The results showed that genetic material of the Zika virus, known as RNA, persisted in fetal brains and in placentas for more than seven months after the mothers contracted Zika.

The RNA levels were about 1,000 times higher in the infants' brains than in the women's placentas, according to the study published in CDC's Emerging Infectious Diseases journal.

The researchers also found evidence of the virus replicating in an infant with microcephaly who died two months after birth.

Specially, they found the Zika virus infects and proliferates in Hofbauer cells, a type of migratory immune cell in the placenta.

Because the Hofbauer cells can move freely throughout the placenta, they may help transfer Zika to the fetus's brain, where the virus can infect various types of brain cells.

Brain tissues from the eight infants with microcephaly who died later were positive for Zika virus. The study showed their mothers and all of the women with positive test results and adverse pregnancy or birth outcomes contracted the Zika virus during their first trimester of pregnancy.

Zika virus RNA was also detected in the placentas of more than one-third of the women who had apparently healthy infants, all of which had Zika infection during their third trimester of pregnancy.

The CDC said that these findings further confirm that Zika virus infection during the first trimester of pregnancy poses more danger for pregnancy and fetal development than infection contracted during the third trimester.

"We don't know how long the virus can persist, but its persistence could have implications for babies born with microcephaly and for apparently healthy infants whose mothers had Zika during their pregnancies," Bhatnagar said. "More studies are needed to fully understand how the virus can affect babies."