In response to the World Health Organization (WHO) statements and international concerns regarding the Zika virus (ZIKV) outbreak, ISUOG is publishing the following guidance for ultrasound during pregnancy. With the current uncertainty regarding many aspects of the diagnosis and clinical course of ZIKV infection in pregnancy, potentially valuable information may be obtained by ultrasound practitioners that may help in counseling pregnant women and further improve our understanding of the pathophysiology of ZIKV infection in pregnancy. There is an outbreak of ZIKV infection in the Americas, Caribbean and South Pacific1, 2. The infection is spread mainly by Aedes mosquitoes, although a small number of cases from sexual transmission have been reported3. The wide distribution of the mosquito, combined with the lack of immunity in the population, has led to rapid evolution of the outbreak. Most cases of ZIKV infection are self-limiting and without sequelae, but there have been cases of Guillain–Barré disease post-infection. In addition, clusters of cases of brain anomalies and microcephaly in some areas with known ZIKV transmission have been reported. This increased number of children with microcephaly has led to a high level of concern among pregnant women living in or traveling to endemic areas. ZIKV can cross the placenta and has been detected using polymerase chain reaction (PCR) analysis of amniotic fluid of pregnancies affected with fetal structural brain abnormalities and microcephaly4, and ZIKV has been isolated postmortem from the brain of a fetus with microcephaly5. A causal relationship between in-utero exposure to ZIKV and microcephaly is now likely, though not yet fully established6. It should be remembered that, for fetal abnormalities to occur due to congenital infection, a number of steps are needed: maternal exposure; maternal infection; fetal infection; and fetal affection. How these steps progress in ZIKV infection is unknown: we do not know how many women exposed in pregnancy become infected, how many of those infected will transmit to the fetus, and what proportion of infected fetuses will suffer effects. It is also important to note that, although microcephaly has been observed, this may well represent the severe end of the spectrum of effects and the co-existence of other abnormalities, while unknown, is likely. The gestational age at which infection occurs is important in other congenital infections, such as cytomegalovirus and toxoplasmosis, and it is probable that ZIKV infection poses the greatest risk in early pregnancy, although effects throughout pregnancy cannot be excluded confidently7. As the situation is evolving rapidly, this guidance will be updated periodically. National guidelines should be followed regarding testing. Expert opinion should be sought from national reference laboratories. In general, testing for ZIKV is possible in maternal serum by reverse transcription PCR (RT-PCR) or detection of ZIKV-specific IgM antibodies8, 9. The limitation of RT-PCR testing is that it can detect ZIKV only during, or immediately following, acute infection. ZIKV IgM testing is problematic because of cross-reactivity with other Flaviviruses and some immunizations. This may lead to an unreliably high false-positive rate of ZIKV serological testing, but negative serology results may be of value in 'ruling out' past ZIKV infection. Expert interpretation of both is required and is beyond the scope of this guidance. In pregnant women with ZIKV exposure and symptoms, positive Flavivirus serology or proven ZIKV infection, or in those with exposure and/or symptoms but who have not had positive serology results, referral for detailed ultrasound assessment is appropriate. A baseline ultrasound scan should be performed on referral. As a minimum this should involve the following. If ultrasound assessment shows a fetal HC of 2 SD below the expected mean for gestational age, or a fetal brain abnormality (such as intracranial calcifications or ventriculomegaly), referral to a specialist center for detailed assessment, including neurosonography of the fetal brain, should be undertaken12. Most fetuses in which the only finding is a HC of 2 SD below the mean would be expected to represent the lower end of the normal population distribution. An interval scan in 2–3 weeks should be arranged14, 15. Given the current uncertainty, existing evidence and experience from prenatal imaging findings in other infections should be taken into account; these include the presence of irregularly shaped ventricular margins, increased periventricular echogenicity with or without cystic lesions, intraventricular adhesions, calcifications, callosal or vermian dysgenesis, small TCD, enlarged cisterna magna and/or increased amount of cerebrospinal fluid around the brain4, 13. Depending on local laws, pregnancy termination may be discussed, based on GA and severity of the findings. Uncertainties regarding the condition should be made clear. Standardized HC measurements should be undertaken and plotted on standards that take into account GA at birth and sex16, 17. The use of a single cut-off regardless of GA is not recommended18. This Interim Guidance was produced by the ISUOG ZIKV Rapid Response Group, members of which are: A. T. Papageorghiou, Fetal Medicine Unit, St George's University Foundation Hospitals NHS Trust, London, and Nuffield Department of Obstetrics and Gynaecology, University of Oxford, Oxford, UK B. Thilaganathan, Fetal Medicine Unit, St George's University Foundation Hospitals NHS Trust, London, UK C. M. Bilardo, Department of Obstetrics, University Medical Centre Groningen, University of Groningen, Groningen, The Netherlands A. Ngu, East Melbourne Ultrasound, East Melbourne, VIC, Australia G. Malinger, Division of Ultrasound in Obstetrics & Gynecology, Lis Maternity Hospital, Tel Aviv Sourasky Medical Center, Sackler Faculty of Medicine, Tel Aviv University, Tel Aviv, Israel M. Herrera, Maternal Fetal Medicine Department, Colombian University Clinic, Colsanitas Clinic, Bogota, Colombia L. J. Salomon, Department of Obstetrics and Maternal-Fetal Medicine, Necker-Enfants Malades Hospital, Assistance Publique-Hôpitaux de Paris, Université Paris Descartes, Paris, France L. E. Riley, Obstetrics and Gynecology, Massachusetts General Hospital, Boston, MA, USA J. A. Copel, Department of Obstetrics, Gynecology and Reproductive Sciences, Yale School of Medicine, New Haven, CT, USA This Interim Guidance should be cited as: 'Papageorghiou AT, Thilaganathan B, Bilardo CM, Ngu A, Malinger G, Herrera M, Salomon LJ, Riley LE, Copel JA. ISUOG Interim Guidance on ultrasound for Zika virus infection in pregnancy: information for healthcare professionals. Ultrasound Obstet Gynecol 2016; 47: 530–532.' Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.