Dr. Margaret Honein and her colleagues at the Centers for Disease Control and Prevention examined 442 cases of women infected with Zika virus. 26 of these pregnancies resulted in Zika-linked birth defects, or about 6 percent. 21 babies were born with defects such as microcephaly, which is a condition where the skull is abnormally small due to improper brain development. The 5 other cases were stillbirths, miscarriages, and abortions. The proportion of pregnancies with birth defects increased to 11 percent when only examining women who were infected with Zika during the first trimester of pregnancy. Women who did did not have noticeable symptoms from Zika had the same rate of birth defects as women who had noticeable symptoms.
The study included foreign women whose pregnancies ended in the U.S., but did not include women in Puerto Rico, an area that currently has a lot of Zika transmission. Almost all women in the study contracted Zika while on travel, or from having sexual activity with someone infected with Zika.