How to diagnose, counsel and manage fetal malformation in one of monochorionic twin pregnancies? [Expert opinion or recommendation] The incidence of fetal malformations in monochorionic twins is 2 to 3 times higher than in singleton pregnancies. Individualized counseling, appropriate monitoring and surgical treatment should be given to pregnant women with monochorionic twin fetuses who develop first trimester anomalies (recommended grade B). Monochorionic twins are 2 to 3 times more likely to have fetal structural abnormalities than singleton pregnancies, such as fetal limb shortage, intestinal atresia, and cardiac malformations, which may be related to abnormal vascular connections between monochorionic twins. Mechanisms such as asymmetric oval sphere division, somatic cell chimerism, and epigenetic modifications can explain the occurrence of many monochorionic twins in which one of the fetuses develops chromosomal abnormalities, neural tube defects, hydrocephalus, and abdominal wall clefts [39]. Once diagnosed, such complicated twins need to be referred for adequate evaluation in an experienced fetal medicine center. The management of monochorionic twins with one fetal anomaly should be individualized by considering the severity of the fetal anomaly, the presence of combined chromosomal abnormalities, the impact on the pregnant woman and the healthy fetus, the risks of the reduction procedure, the patient’s wishes, and ethical and social factors [40]. If the decision is made to reduce the fetus, the approach is the same as that of the reduction procedure for sIUGR (level of evidence III).