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Health condition

Conjoined twins, classification, antenatal diagnosis and specialist care.

An extremely rare condition, and one of the most complex journeys in medicine. This guide sets out how it is diagnosed and where expert care sits in the UK and internationally.

A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Editorial team

    Written by our editorial team from the UK sources listed on each page.

  • 02

    Sourced from guidance

    Checked against RCOG, GOSH and peer-reviewed fetal medicine sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK fetal medicine practice including antenatal MRI, echo characterisation and specialist referral pathways.

Key facts

Conjoined twins at a glance.

The essentials, in plain English - what it is, how it is classified, and where specialist care sits in the UK.

  • What it is

    Extremely rare condition where monochorionic monoamniotic twins fail to fully separate in early embryogenesis, resulting in physical union.

  • How rare

    Estimated incidence of roughly 1 in 50,000 to 1 in 100,000 births, with a strong female predominance of around 70 per cent.

  • Survival

    A large proportion are stillborn or die shortly after birth. Outcome depends heavily on which organs are shared.

  • Classification

    Named by the point of union - thoracopagus, omphalopagus, pygopagus, ischiopagus, craniopagus and rarer patterns.

  • Diagnosis

    Antenatal ultrasound at the 20-week anomaly scan or earlier, with fetal MRI and echocardiography for detailed anatomy.

  • Care

    Highly specialist multidisciplinary care - Great Ormond Street Hospital is an internationally recognised centre for separation surgery.

Why this guide matters

Extremely rare, extremely specialist.

Fewer than one in 50,000 pregnancies. Care sits in a handful of tertiary centres. The three points below shape everything on this page.

  • Anatomy decides everything

    Which organs are shared - especially the heart - is the single most important factor in prognosis and whether separation is feasible.

  • Imaging is the foundation

    Antenatal ultrasound, fetal MRI and detailed echocardiography build the map that every subsequent decision rests on.

  • Expertise is concentrated

    Only a small number of centres worldwide, including Great Ormond Street Hospital, have deep experience of separation surgery.

How the diagnosis is made

From first scan to a specialist plan.

The steps a UK fetal medicine team will normally follow, in order - so families know what to expect and why.

  1. 01

    Detecting

    First-trimester ultrasound

    Some cases are picked up on early scans when monoamniotic twins are seen sharing tissue or failing to move independently.

  2. 02

    Detecting

    20-week anomaly scan

    The routine mid-trimester scan is when most cases are formally identified and classified.

  3. 03

    Detecting

    Fetal medicine referral

    Immediate referral to a tertiary fetal medicine unit for expert scanning, counselling and planning.

  4. 04

    Characterising

    Detailed fetal echocardiography

    Cardiac anatomy is the single most important prognostic factor - a specialist fetal cardiologist maps every chamber and vessel.

  5. 05

    Characterising

    Fetal MRI

    MRI complements ultrasound with high-resolution soft-tissue detail, essential for characterising shared organs and planning surgery.

  6. 06

    Planning

    Multidisciplinary planning

    Fetal medicine, neonatology, paediatric surgery, anaesthesia and ethics teams meet to plan delivery and postnatal care.

  7. 07

    Planning

    Delivery at a specialist centre

    Planned caesarean at a tertiary hospital with immediate access to paediatric surgical and intensive care support.

Typical timeline: detection at the 20-week scan, then months of specialist characterisation and planning.

Classification

Named by the point of union.

The six major patterns and the rarer variants - and why classification drives every subsequent decision.

  • Thoracopagus

    Around 40 per cent of cases - joined at the chest, typically sharing heart and pericardium. The most common and often the most complex pattern.

  • Omphalopagus

    Around 33 per cent - joined at the abdomen, often sharing liver, bowel or biliary tree, but usually with separate hearts.

  • Pygopagus

    Around 19 per cent - joined at the sacrum and buttocks, often sharing lower spine, pelvis or bowel.

  • Ischiopagus

    Around 6 per cent - joined at the pelvis, often sharing lower urinary tract, genitalia and pelvic bones.

  • Craniopagus

    Around 2 per cent - joined at the cranium, sometimes sharing dura, venous sinuses or brain tissue. Among the most technically demanding to separate.

  • Parapagus and rachipagus

    Rarer patterns - side-by-side union along the trunk (parapagus) or along the spine (rachipagus), each with its own anatomy.

  • Shared organs

    The nature and number of shared organs is the single greatest determinant of whether separation is feasible.

  • Red flag - shared heart

    A shared or fused heart dramatically limits separation options and prognosis, and needs early expert counselling.

Management

How conjoined twins are cared for.

From fetal medicine and delivery planning to separation surgery, reconstruction and long-term support - a genuine multidisciplinary undertaking.

  • Specialist fetal medicine

    Expert scanning, counselling and options review at a tertiary unit, including full information about continuing the pregnancy or termination.

  • Delivery planning

    Planned caesarean at a highly specialist paediatric surgical centre, with neonatal intensive care and surgical teams ready from birth.

  • Neonatal stabilisation

    Immediate resuscitation, airway support and imaging in a paediatric intensive care setting, with a plan for feeding and shared physiology.

  • Separation surgery

    Highly complex, often staged operations performed at a handful of global centres. Involves months of planning, tissue expansion and rehearsal.

  • Tissue expansion

    Skin expanders are placed weeks or months before separation to grow enough skin to close both babies after division.

  • Reconstructive surgery

    Plastic, orthopaedic, urological and general surgeons rebuild abdominal walls, chest walls, pelvis and genitourinary tracts after separation.

  • Non-separation pathway

    Where separation is not feasible or safe, families are supported to live with shared existence, with specialist medical, educational and psychological input.

  • Great Ormond Street pathway

    GOSH has an international reputation for conjoined twin care, working with fetal medicine centres across the UK and abroad.

What this guide is based on

The sources behind every claim on this page.

UK and international specialist guidance and peer-reviewed sources, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your fetal medicine team knows the anatomy of your pregnancy and can tell you which parts apply to you. Always follow their advice.

  • Royal College of Obstetricians and Gynaecologists (RCOG). Management of monochorionic twin pregnancy (Green-top Guideline).

  • Great Ormond Street Hospital. Information on conjoined twin assessment and separation surgery.

  • International Society of Ultrasound in Obstetrics and Gynecology (ISUOG). Practice guidelines on twin pregnancy.

  • Spitz L, Kiely EM. Experience in the management of conjoined twins. British Journal of Surgery.

Red flags

The features that raise the stakes.

The anatomical, ethical and logistical factors that change what is possible and where care needs to happen.

  • Shared or fused heart

    A shared cardiac chamber or single heart is the most powerful negative prognostic factor and needs urgent specialist fetal cardiology input.

  • Craniopagus with shared venous sinuses

    Cranial union involving shared dural venous drainage is among the most technically hazardous patterns to separate.

  • Complex hepatobiliary sharing

    Fused liver with shared biliary tree needs detailed MRI mapping and highly specialist hepatobiliary planning.

  • Severe associated anomalies

    Additional major cardiac, neurological or airway anomalies significantly alter feasibility and outcome.

  • Acute deterioration of one twin

    If one twin becomes critically unwell, an emergency separation may be considered to save the other - a devastating and highly specialist decision.

  • Airway or respiratory compromise

    Thoracic union can restrict lung expansion and complicate ventilation, requiring specialist paediatric anaesthetic input from birth.

  • Ethical and consent complexity

    Decisions around separation, non-separation and sacrifice of one twin need formal ethics review and, at times, court oversight.

  • Family psychological distress

    Parents, siblings and extended family need long-term psychological support alongside the clinical care.

  • Limited global expertise

    Only a small number of centres worldwide have deep experience - timely referral to one of them matters.

Living with it

A journey, with a team behind every step.

Four things that shape family life through pregnancy, surgery and the years afterwards - a coordinated team, a long staged journey, charity support and the freedom to choose.

A quiet reminder

There is no single right path.

Every family faces different anatomy, different beliefs and different circumstances. Specialist teams are there to support the choice that is right for you.

  1. 01 Team

    A truly multidisciplinary team

    Paediatric surgery, neonatology, anaesthesia, plastic surgery, orthopaedics, urology, genetics, psychology and ethics all sit at the same table.

  2. 02 Journey

    A long, staged journey

    Care spans pregnancy, delivery, months of planning, surgery and years of follow-up. Expectations are set carefully at every step.

  3. 03 Support

    Charity and family support

    Great Ormond Street Hospital Charity and specialist family support services help with the practical, emotional and financial burden.

  4. 04 Choices

    Every family is different

    Some families proceed with separation, some do not. Both paths are supported with dignity and specialist input.

Frequently asked

Everything we get asked about conjoined twins.

Quick answers on incidence, classification, diagnosis and where specialist separation surgery is performed.

  • What are conjoined twins?

    Conjoined twins are an extremely rare form of monochorionic monoamniotic twinning where the embryos fail to fully separate in early pregnancy, resulting in physical union at some point on the body. Incidence is estimated at roughly 1 in 50,000 to 1 in 100,000 births and there is a strong female predominance.

  • How are conjoined twins classified?

    They are named by the anatomical point of union. The commonest patterns are thoracopagus (chest, around 40 per cent), omphalopagus (abdomen, around 33 per cent) and pygopagus (sacrum and buttocks, around 19 per cent). Rarer patterns include ischiopagus (pelvis), craniopagus (cranium), parapagus and rachipagus.

  • How is the diagnosis made?

    Most cases are identified on antenatal ultrasound, often at the routine 20-week anomaly scan or earlier. Detailed fetal echocardiography and fetal MRI are essential to characterise shared organs and plan care. All cases are referred to a tertiary fetal medicine centre.

  • Can conjoined twins be separated?

    It depends entirely on which organs are shared. Some patterns, particularly those with a shared heart or complex cranial venous drainage, may not be safely separable. Others can be separated through highly complex, often staged surgery at a small number of specialist centres worldwide, including Great Ormond Street Hospital in London.

  • Where is separation surgery performed?

    Separation is only performed at a handful of centres with deep experience, including Great Ormond Street Hospital in the UK, a small number of specialist units in the United States, and centres in Saudi Arabia and elsewhere. Cases are often referred internationally to a team with the specific expertise required.

  • What happens if separation is not possible?

    Some conjoined twins live full lives without separation, with specialist medical, educational and psychological support. Families are supported to make choices that fit their beliefs and circumstances, with input from paediatric teams, ethics services and family support charities.

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