Conjoined Twins
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Direct answer
Conjoined twins are monozygotic twins whose bodies are joined because the embryo split incompletely, after about day 13 of fertilisation. They are always monochorionic and monoamniotic. Antenatal diagnosis relies on ultrasound showing fusion, a single amniotic sac and fixed shared structures, with MRI and fetal echocardiography defining the anatomy. Thoracopagus and omphalopagus are the commonest types.
What you must remember
- Origin: Incomplete splitting of the embryo after about day 13, so conjoined twins are always monochorionic and monoamniotic.
- Classification: Named by the site of fusion, such as thoracopagus, omphalopagus, craniopagus, pygopagus and ischiopagus.
- Commonest types: Thoracopagus and omphalopagus, which together account for most cases.
- Early suspicion: Raised on ultrasound at the first or second trimester when the twins are fixed, face each other or share a single sac.
- Key investigations: Ultrasound with colour Doppler, fetal echocardiography and MRI to define shared organs such as the heart, liver and brain.
- Prognosis: Depends mainly on shared vital organs, particularly the brain and heart, and on whether separation is feasible.
- Legal counselling: Under the Indian MTP Act as amended, the 24-week limit does not apply when a Medical Board diagnoses a substantial fetal abnormality, and the board decides on termination.
Classification by the site of fusion
Conjoined twins are classified by the anatomical site of fusion, which shapes both surgical options and prognosis. The table lists the common types and the organs most often shared.
| Type | Site of fusion | Key note |
|---|---|---|
| Thoracopagus | Chest and upper abdomen | Commonest type, often shares the heart |
| Omphalopagus | Lower chest and abdomen | Often shares the liver |
| Craniopagus | Skull and brain | Prognosis depends on shared brain vessels |
| Pygopagus | Lower back and buttocks | Fused at the sacrum |
| Ischiopagus | Pelvis | Shares pelvic structures and lower bowel |
Delivery planning depends on the site of fusion, the presentation and the size of the shared structures, so the plan should be made in a specialist unit with paediatric surgical and neonatal support.
Counselling involves a multidisciplinary team, including paediatric surgery, neonatology, anaesthesia and ethics or legal input. The family needs time to understand the options before a decision is made. The plan is documented early so the delivery and neonatal teams are ready.
Where the exam tries to catch you
Questions test the chorionicity, the classification and the factors that decide prognosis.
- Trap: Calling conjoined twins dichorionic. Correct: Conjoined twins are monochorionic and monoamniotic, because the split occurred late in embryonic development.
- Trap: Assuming conjoined twins always need caesarean section. Correct: The route is individualised, and vaginal delivery can be considered in some cases depending on size, fusion site and presentation.
- Trap: Judging prognosis by the number of fused body parts. Correct: Prognosis depends mainly on the shared vital organs, particularly the brain and heart.
- Trap: Planning separation before imaging is complete. Correct: MRI and echocardiography define the shared organs before any surgical plan is made.
Frequently asked questions
What is the most common type of conjoined twins?
Thoracopagus is the most common type of conjoined twins, in which the twins are joined at the chest and upper abdomen. Omphalopagus, joined at the lower chest and abdomen, is also common. Prognosis depends more on the shared heart, liver and other organs than on the site alone.
When is conjoined twinning usually detected antenatally?
Conjoined twins are usually detected on ultrasound in the first or second trimester, when fixed positions, a single amniotic sac and shared structures are seen. A detailed anomaly scan improves detection. MRI and fetal echocardiography then define the anatomy.
What investigations define the anatomy of conjoined twins before delivery?
Ultrasound with colour Doppler, fetal echocardiography and MRI are the main investigations used to define shared organs in conjoined twins. Their findings determine prognosis and whether separation is feasible. Delivery planning follows from these results.
Does conjoined twinning always mean delivery by caesarean section?
Conjoined twins do not always require caesarean delivery. The route depends on presentation, the site of fusion, size and obstetric factors, and a specialist team decides it. Caesarean delivery is often chosen when fusion is extensive or obstructed labour is expected.