This paper reports the ultrasound diagnosis at 16 weeks' gestation of thoracopagus conjoined twins. Ultrasound examination showed the two fetuses conjoined at the sternum, with a single heart and a single liver. After informing the couple of the extremely poor prognosis, medical termination of pregnancy was requested. Pathologic examination of the conjoined female fetuses revealed a single, non-duplicated heart, two livers connected at the right lobe, completely separate bile ducts and digestive tract, and a single placenta and umbilical cord containing two arteries and six veins. The karyotype was normal. Diagnostic ultrasound criteria for thoracopagus conjoined twins include: the relative position of the two fetuses facing each other, hyperextension of the cervical spine, continuity of the skin and mirror image body parts with limbs close together. The presence of a single heart, liver and umbilical cord, all of increased size, confirms the diagnosis. Various degrees of fetal fusion result from incomplete division of the inner cell mass 13 to 15 days after fertilization. Although the precise causes are unknown, many workers believe that the factors responsible for monozygosity may also play a role in conjoined twins. In Switzerland, 1% of all live births are twins with approximately 1/4 of these monozygotic. If incomplete division of the inner cell mass occurs in 1% of these cases, the estimated incidence of conjoined twins is 1/40000 births. Although thoracopagus twins are more frequent, omphalopagus twins are more commonly encountered at birth, due to lower fetal mortality. The overall prognosis depends on the degree of organ sharing between fetuses. Very few surgically separated thoracopagus conjoined twins have lived and those who did survive had separate hearts. Also, conjoined twins can cause dystocia with the risk of rupture of the uterus, and often require cesarean section which may have negative consequences for the obstetrical future of the mother. However, an early ultrasound diagnostic can modify prognosis and allow medical termination of pregnancy in the case of seriously malformed thoracopagus conjoined twins. The risk that the condition recurs in a subsequent pregnancy may be considered negligible.