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American mothers infected with the Zika virus last year were 20 times as likely to give birth to babies with birth defects as mothers who gave birth two years before the epidemic, federal health officials said on Thursday. The Centers for Disease Control and Prevention concluded last April that Zika infection caused severe birth defects, including the abnormally small heads of microcephaly, but it had not previously estimated how common such defects were. (McNeil, 3/2)

On the heels of a new study from the Centers for Disease Control and Prevention showing that Zika has increased the rate of birth defects in the United States, Florida health officials on Thursday reported three more cases of the virus that were locally acquired in Miami-Dade — two infections dating to October 2016 and the first one of 2017. The two Zika cases from 2016 required confirmatory testing from the Centers for Disease Control and Prevention, the health department said in a press release announcing the cases. The third case involved a person who had no symptoms but donated blood in January, leading to the discovery of a past infection. (Chang, 3/2)

Researchers at the Centers for Disease Control and Prevention are trying to determine how common these birth defects, such as microcephaly, brain abnormalities, eye defects and central nervous system problems, were in the years before the Zika outbreak. Although a Zika infection during pregnancy is linked to a distinct pattern of birth defects, those abnormalities are not unique to Zika. Genetic factors and other viral infections may also cause these birth defects, although in many cases the causes are unknown, experts say. (Sun, 3/2)

The upshot: For a pregnant woman outside of areas where mosquitoes are spreading the Zika virus, the risk of having (or losing) a baby with a defect such as microcephaly, neural tube defect, or brain or eye abnormality is pretty low, well under 1%. (Healy, 3/2)