The STAT website first reported the U.S. miscarriages, based on information from the CDC's chief pathologist. The pathologist told STAT the women miscarried early in their pregnancies but provided no additional details.

Last month, officials said a baby born in a Hawaii hospital was the first in the country with a birth defect linked to Zika. Hawaii officials said the baby's mother likely contracted the virus while living in Brazil last year and passed it on while her child was in the womb. Babies born with this rare condition, known as microcephaly, have abnormally small heads and brain abnormalities.

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In cases when women have one or two miscarriages, the cause is usually severe chromosomal problems, experts say. "It's absolutely possible for an infection, whether it be viral or bacterial, to result in a miscarriage," said Zev Williams, an obstetrician-gynecologist who specializes in pregnancy loss at the Albert Einstein College of Medicine at Montefiore Medical Center in New York. "Whether it was caused by Zika remains to be determined," he said, but urged individuals to take precautions to avoid contracting or transmitting the virus.

Some virus infections in pregnancy, like Rubella or German measles infections especially early in pregnancy, can spread from the mother and infect the cells of the fetus and cause direct injury to it, said Jesse Goodman, an infectious diseases doctor at Georgetown University.

In testimony before Congress Wednesday, CDC Director Tom Frieden reiterated that the agency is learning more about Zika every day, including how it can be transmitted from mother to fetus. Increasing evidence in Brazil also is linking Zika to microcephaly and other suspected neurological complications.

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More than four dozen Zika cases have been confirmed in 14 states and the District of Columbia -- six involving pregnant women -- with at least another 21 cases in U.S. territories, the CDC said last Friday. Frieden also said that one U.S. case of Guillain-Barré syndrome may be linked to Zika.

It was unclear whether the two miscarriages were counted among the six cases involving pregnant women. Global health officials are closely monitoring the spread of the virus and the incidence of suspected neurological complications. Frieden has said the link between Zika and Guillain-Barré, which can lead to paralysis in adults, is growing stronger. Several South American countries have identified cases of the syndrome.

The World Health Organization, which has designated the outbreak a "global public health emergency," issued guidance Wednesday on how women should protect themselves against possible sexual transmission of Zika. It said that until more is known, "all men and women living in or returning from an area where Zika is present -- especially pregnant women and their partners -- should be counseled on the potential risks of sexual transmission and ensure safe sexual practices."

Those include the correct and consistent use of condoms, the WHO said.

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Last week the CDC issued its own detailed recommendations for preventing sexual transmission of the virus, including the suggestion that men who have traveled to Zika-affected regions consider abstaining from sex with their pregnant partner for the duration of the pregnancy. The guidelines came after a Dallas resident was infected by having sex with a person who had contracted the disease while traveling in Venezuela.