An acardiac twin is one of the rarest and most serious birth defects known to medicine. It occurs when one twin develops without a heart — or with only a very incomplete heart — and depends entirely on its sibling to survive in the womb. This condition affects roughly one in every 35,000 pregnancies. In this article, we explain what causes it, how it is diagnosed, what risks it poses, and what Canadian families can expect from diagnosis through delivery.
What Is an Acardiac Twin?
The term “acardiac” comes from the Greek word meaning “without a heart.” In a normal twin pregnancy, each baby grows its own heart and circulatory system. However, in an acardiac twin pregnancy, one twin — called the acardiac twin — fails to develop a functioning heart at all.
Because it has no heart, the acardiac twin cannot pump blood on its own. Instead, it receives blood from its co-twin, known as the “pump twin.” The pump twin does all the circulatory work for both babies. This places enormous strain on the pump twin’s heart.
This condition is also known as Twin Reversed Arterial Perfusion (TRAP) sequence. The name describes exactly what happens: blood flows in reverse through shared vessels, travelling from the pump twin into the acardiac twin. You can read more about rare fetal conditions through Health Canada’s reproductive health resources.
How Does an Acardiac Twin Develop?
TRAP sequence only occurs in identical (monozygotic) twin pregnancies. These are pregnancies where one fertilised egg splits into two embryos. The condition does not occur in fraternal twins.
Early in development, the two embryos share a placenta. Abnormal blood vessel connections form between them. As a result, blood from the pump twin flows backwards into the acardiac twin.
Because the acardiac twin receives poorly oxygenated blood — and receives it in reverse flow — its upper body often fails to develop properly. In many cases, the acardiac twin has no recognisable head, heart, or arms. The lower body may be partially formed, sometimes with legs and a pelvis. This is sometimes called a fetus acardius.
Why Does This Happen?
Researchers believe the root cause is the abnormal formation of blood vessel connections in a shared placenta. These connections redirect circulation in a way that one embryo becomes dominant. The other embryo essentially becomes dependent on the first for all blood flow.
This is not caused by anything the mother did or did not do during pregnancy. There are no known lifestyle, dietary, or environmental factors that cause TRAP sequence. It is considered a random developmental event.
How Common Is the Acardiac Twin Condition?
TRAP sequence is extremely rare. It occurs in approximately 1 in 35,000 pregnancies overall. However, among identical twin pregnancies specifically, the rate rises to about 1 in 100.
In Canada, where multiple pregnancies have increased partly due to assisted reproductive technologies (ART) such as IVF, awareness of rare twin complications has grown. However, because TRAP sequence only affects identical twins — not fraternal twins created through IVF — the rise in IVF use has not directly increased its rate.
Furthermore, improved prenatal ultrasound screening across Canadian provinces means the condition is now detected earlier than ever before. Most cases are identified during routine first- or second-trimester ultrasounds.
Risks to the Pump Twin
The acardiac twin itself cannot survive outside the womb. It has no functioning heart and no independent circulatory system. However, the greater medical concern is the serious risk the condition poses to the pump twin — the healthy sibling doing all the circulatory work.
Heart Failure in the Pump Twin
The pump twin’s heart must work much harder than a normal fetal heart. It supplies blood to itself and to the acardiac twin simultaneously. Over time, this extra workload can lead to high-output cardiac failure — a condition where the heart is overworked to the point of failing.
Without treatment, the pump twin faces a mortality rate of up to 50 to 75 percent. This is why early diagnosis and careful monitoring are so important. The Mayo Clinic’s overview of high-risk pregnancies outlines how fetal cardiac stress is managed in complex twin pregnancies.
Preterm Birth and Other Complications
The acardiac twin’s body continues to grow throughout pregnancy, even without a heart. As it grows, it demands more blood from the pump twin. This can cause polyhydramnios — an excess of amniotic fluid — which significantly increases the risk of preterm labour.
Other risks to the pump twin include:
Preterm delivery, often before 34 weeks
Heart enlargement due to increased workload
Hydrops fetalis — a serious condition involving fluid build-up in the baby’s body
Intrauterine death if the condition is left untreated
Diagnosis: How Is an Acardiac Twin Detected in Canada?
In most cases, an acardiac twin is first suspected during a routine prenatal ultrasound. Canadian provincial health plans cover standard prenatal ultrasounds, typically at around 18 to 20 weeks. However, the condition can sometimes be spotted as early as the first trimester with advanced imaging.
Key signs visible on ultrasound include:
One twin with no visible heartbeat or cardiac activity
A poorly formed or absent upper body in one twin
Reversed blood flow in shared placental vessels (detected with Doppler ultrasound)
Signs of cardiac stress or enlargement in the pump twin
Specialist Referral and Monitoring
Once TRAP sequence is suspected, your family doctor or obstetrician will refer you to a maternal-fetal medicine (MFM) specialist. These doctors specialise in high-risk pregnancies. In Canada, MFM specialists are available at major hospitals in cities such as Toronto, Vancouver, Calgary, Montreal, and Ottawa.
Ongoing monitoring typically includes frequent Doppler ultrasounds to track blood flow, fetal echocardiograms to assess the pump twin’s heart, and careful measurements of both twins’ growth.
Treatment Options for TRAP Sequence
There is no cure for the acardiac twin. The acardiac twin cannot survive. Treatment therefore focuses entirely on protecting the pump twin and giving it the best possible chance of a healthy delivery.
Watchful Waiting
In mild cases — where the acardiac twin is small and the pump twin shows no signs of cardiac stress — doctors may choose careful observation. Frequent ultrasounds monitor for any signs of deterioration. Some pregnancies reach full term without intervention.
Minimally Invasive Procedures
In more serious cases, doctors may recommend a procedure to stop blood flow to the acardiac twin. By cutting off its blood supply, the pump twin’s heart is relieved of its extra workload. Common procedures include:
Radiofrequency ablation (RFA) — uses heat energy delivered through a needle to block blood vessels in the acardiac twin
Laser coagulation — uses laser energy to seal off shared blood vessels
Cord coagulation — seals the umbilical cord of the acardiac twin
These procedures are performed by specialist fetal medicine teams. In Canada, they are typically available at tertiary care centres affiliated with major university hospitals. According to Healthline’s fetal medicine resources, radiofrequency ablation has shown strong success rates in improving pump twin survival.
After Delivery
If the pump twin survives to delivery, it can lead a fully healthy life. The acardiac twin is typically delivered alongside the pump twin but does not survive outside the womb. Families are usually offered grief counselling and emotional support after delivery.
When to See a Doctor
If you are pregnant with identical twins, it is important to attend all scheduled prenatal appointments. Most provincial health plans cover regular prenatal care, including ultrasounds that can detect this condition early.
Contact your family doctor or go to a walk-in clinic right away if you notice:
Sudden or rapid increase in abdominal size
Decreased movement from one or both babies
Signs of preterm labour, such as regular contractions before 37 weeks
Unusual pressure or discomfort in the abdomen
If you have already been diagnosed with a twin pregnancy and have not yet had a detailed anatomy ultrasound, ask your family doctor for a referral. Early detection is key to giving the pump twin the best possible outcome. Always speak with a qualified healthcare provider before making any decisions about your pregnancy care.
Frequently Asked Questions About Acardiac Twin
Can an acardiac twin survive?
No, an acardiac twin cannot survive. It has no functioning heart and cannot sustain life outside the womb. All medical treatment focuses on protecting the healthy pump twin so it can survive and thrive after birth.
What causes TRAP sequence or acardiac twin pregnancy?
TRAP sequence is caused by abnormal blood vessel connections in a shared placenta between identical twins. These connections cause blood to flow in reverse into one twin, preventing its heart from developing. It is a random developmental event and is not caused by anything the mother does during pregnancy.
How is an acardiac twin diagnosed?
An acardiac twin is most often diagnosed during a prenatal ultrasound, when one twin shows no heartbeat and reversed blood flow is detected through Doppler imaging. In Canada, this is typically picked up during routine provincial health plan ultrasounds in the first or second trimester. A maternal-fetal medicine specialist will then confirm the diagnosis.
What is the survival rate for the pump twin in an acardiac twin pregnancy?
Without treatment, the pump twin faces a mortality risk of up to 50 to 75 percent. However, with early diagnosis and procedures such as radiofrequency ablation, survival rates for the pump twin improve significantly. Many pump twins go on to be born healthy and live completely normal lives.
Is acardiac twin the same as TRAP sequence?
Yes, the two terms describe the same condition. TRAP stands for Twin Reversed Arterial Perfusion, which describes the mechanism behind the acardiac twin condition — blood flows in reverse from the healthy twin into the twin without a heart. Doctors may use either term when discussing this diagnosis.
Can an acardiac twin pregnancy happen with IVF?
TRAP sequence only occurs in identical twin pregnancies, not fraternal twin pregnancies. Since IVF typically produces fraternal twins, the procedure does not directly increase the risk of an acardiac twin pregnancy. However, if IVF results in identical twins — which is rare — the same baseline risk applies as with any identical twin pregnancy.
Key Takeaways
An acardiac twin is a rare condition where one twin develops without a functioning heart, affecting about 1 in 35,000 pregnancies.
Also called TRAP sequence, the condition only occurs in identical twin pregnancies where both babies share a placenta.
The acardiac twin cannot survive, but the pump twin — the healthy sibling — can survive with early diagnosis and proper treatment.
Without treatment, the pump twin faces a high risk of heart failure and death due to the extra circulatory workload.
Treatment options include watchful waiting for mild cases, or minimally invasive procedures such as radiofrequency ablation for more serious cases.
In Canada, diagnosis typically occurs through routine prenatal ultrasounds covered by provincial health plans, followed by referral to a maternal-fetal medicine specialist.
If you are pregnant with identical twins, attend all prenatal appointments and speak with your family doctor about any concerns as early as possible.
