- 1% of monochorionic twins.
- Results from incomplete splitting of the embryonic mass after day 12 of fertilization.
- 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 180o angle), craniophagus (fused skulls and separate bodies), omphalopagus (fused at the lower abdomen, with shared liver and intestines).
- The incidence of chromosomal abnormalities and genetic syndromes is not increased.
- Detailed ultrasound examination and assessment by multidisciplinary team.
- 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.
- Very high risk of handicap in survivors.
- No increased risk of recurrence.