Pregnant women without Zika virus symptoms who have lived in or traveled to areas with Zika transmission should receive nucleic acid testing for Zika at least once per trimester, in addition to the previously recommended immunoglobulin M testing, the Centers for Disease Control and Prevention said in an advisory today. “If positive, this may provide a more definitive diagnosis of recent Zika infection,” CDC said. “However, a negative NAT does not rule out recent infection because viral ribonucleic acid declines over time. Other diagnostic methods, such as NAT testing of amniocentesis specimens or serial ultrasounds, may provide additional information to help determine whether the IgM test results suggest a recent infection.” Recommendations for testing symptomatic pregnant women remain unchanged, the agency said.