Five of nine pregnancies among U.S. women who were infected with the Zika virus have resulted in tragic outcomes, federal health officials said Friday.

All of the women contracted the mosquito-borne virus while traveling outside the United States, in regions experiencing Zika outbreaks, the officials said.

READ: Zika Virus Expected To Spread; CDC Issues Increased Travel Alert

In four of the cases, the women lost their babies: Two to miscarriage and two to abortions after ultrasounds revealed birth defects, the U.S. Centers for Disease Control and Prevention reported.

A fifth woman gave birth in late 2015 to a child with severe microcephaly, a condition in which the brain and skull are significantly underdeveloped.

Since the Zika epidemic began last spring, it’s believed there have been more than 5,600 suspected or confirmed cases of microcephaly in Brazil, the World Health Organization reported Friday.

“Even though the [U.S.] numbers are small, they are of considerable interest,” CDC Director Dr. Tom Frieden said during a news conference Friday. “We understand that the occurrence of fetal malformation, fetal loss or miscarriage, or a child with a birth defect, is something that can be devastating to a family.”

READ: 4 Reasons You Are Mosquito Bait

The proportion of U.S. pregnancies that appear to have been harmed due to the mosquito-borne infection “is unexpected, and greater than we would have expected,” Dr. Denise Jamieson, co-lead of the Pregnancy and Birth Defects Team with the CDC Zika Virus Response Team, said during the news conference.

Of the remaining four women, two gave birth to apparently healthy babies and two pregnancies are continuing without known complications.

Those numbers reflect confirmed cases as of Feb. 17. Ten additional reports of Zika infection involving pregnant U.S. women are currently under investigation, the CDC added.

Genetic evidence of Zika virus was detected in tissue specimens from the two miscarriages. The CDC was careful to say it is not known whether Zika caused the miscarriages, but Frieden noted that the presence of the virus in fetal tissue is highly suspicious.

In an updated travel alert issued Friday evening, the CDC offered several cautions for people planning to attend the 2016 Summer Olympics in Rio de Janeiro, Brazil. The agency suggested that pregnant women consider not going to the games and that if their male partners go, the couple should use condoms after the trip or abstain from sex during the pregnancy.

If a pregnant women decides to go to the Summer Olympics, she should talk with her doctor before leaving, the CDC advised in the latest alert.

Join the conversation and share this story