Prevalence:

• 1% of monochorionic twins.
• Results from incomplete splitting of the embryonic mass after day 12 of fertilization.

Ultrasound diagnosis:

• Fused twins in monochorionic monoamniotic pregnancy.
• Classified according to the site of fusion followed by pagus (Greek word for 'what is fixed'). Most common type is thoracopagus (75% of cases) with fusion in the thorax and abdomen and often conjoined hearts, livers and intestines.
• Other types include pygopagus (fetuses fused at the rump), ischiopagus (fetuses fused at the lower half of the bodies with spines conjoined end to end at 180^o angle), craniophagus (fused skulls and separate bodies), omphalopagus (fused at the lower abdomen, with shared liver and intestines).

Associated abnormalities:

• The incidence of chromosomal abnormalities and genetic syndromes is not increased.

Investigations:

• Detailed ultrasound examination and assessment by multidisciplinary team.

Management:

• Pregnancy termination, stillbirth or neonatal death in >90% of cases.
• If the pregnancy continues, delivery should be by cesarean section in a centre with expertise in the management of such pregnancies.

Prognosis:

• Very high risk of handicap in survivors.

Recurrence:

• No increased risk of recurrence.