[Ultrasonography of twin pregnancies]

Sonographers must be familiar with the diagnostic tools available to them for determining twin pregnancy type. During the first trimester, these tools include the number of chorionic sacs, the number of yolk sacs, and the presence of an inter-twin membrane. During the second trimester, sonographers must look for a difference in sex, the presence of two separate placentas, and a thick vs. thin membrane, or the absence of a membrane. Complications are frequent in twin pregnancies and may be maternal or fetal. Measuring cervical length is the only way for sonographers to predict pre-term labour. A cervical length of< or =2.5 cm before 23 weeks is significant. Some fetal anomalies are more frequent in twin pregnancy than in single pregnancy. The more frequent are: esophageal atresia, omphalocele and cardiac anomalies. Some fetal anomalies are peculiar to twin pregnancy: Siamese twins, digestive atresia, craniofacial deformation and club-foot. A significant growth asymmetry is defined as a CRL of 3 or more millimetres and a difference of 20% in weight. The poly-oligo sequence represents a severe level asymmetry of amniotic fluid in monodiamniotic twins where survival rate without sequelae is 13.6%. The twin-twin transfusion syndrome is an event occurring after the apparition of the poly-oligo sequence, which manifests by discordances in weight and hemodynamic changes leading to myocardial repercussion. Twin embolization syndrome follows the in utero death of a co-twin in a monochorionic twin pregnancy; ischemic events must be monitored in such cases. Cord entanglement is unpredictable and is specific to monochorionic monoamniotic twin pregnancies. Finally, an acardius is a parasitic twin that feeds on its co-twin, resulting in the viable twin's death in 50% of cases.