Zika virus is a Flavivirus spread predominantly by Aedes mosquitoes. Acute infections can occur with mosquito bites or through sexual transmission and can manifest with low-grade fever, rash, arthralgia, conjunctivitis, and headache. In pregnant women, acute Zika infection has been associated with fetal death and fetal brain defects such as microcephaly, intracranial calcifications, and cerebral malformation, as well as ophthalmologic lesions. In a prospective cohort study, investigators enrolled 345 pregnant women at any period of gestation in Rio de Janeiro from September 2015 through May 2016 who had presented to clinic with rash that had developed within the previous five days. The investigators determined pregnancy and infant outcomes for the women with acute Zika infection. At the time of enrollment, blood and urine samples were tested for Zika virus. Patient specimens were also tested for viruses that can cause fever, rash, or congenital defects, such as dengue, chikungunya, parvovirus b19, and cytomegalovirus, as well as for syphilis and acute HIV infection. All study participants received regular weekly telephone follow-up along with a second clinic visit within 30 days of enrollment and serial fetal ultrasonography during the pregnancy. One hundred and eighty-two of the 345 pregnant women, or 53%, tested positive for Zika virus by PCR in blood, urine, or both. The timing of acute Zika infection ranged from six to 39 weeks of gestation, with more than half of the infections occurring in the second trimester. The majority of women infected with Zika virus had pruritic rash, arthralgia, headache, and conjunctival symptoms. Over 25% had a low-grade fever. By the end of July 2016, a total of 186 pregnancies — 125 affected by Zika virus, and 61 unaffected — had reached completion and were fully evaluated. One hundred and seventeen live births occurred in Zika virus-affected pregnancies and 57 live births in unaffected pregnancies. The fetal mortality rate was seven percent in both groups, 49 (or 42) percent of the live births in the Zika virus affected pregnancies had evidence of gross central nervous system abnormalities on clinical exam or brain imaging, as compared with three (or five) percent of the live births in the pregnancies unaffected by Zika virus. Microcephaly was observed in four infants whose mothers were infected with Zika virus in weeks 8, 12, 30, and 38 of gestation. In conclusion, acute Zika virus infection during pregnancy can have harmful fetal consequences with over 40% of infants having evidence of abnormal CNS findings during the first months of life. Full research report findings are available at NEJM.org.