What Causes Trisomy 18?

Trisomy 18 usually happens due to a random error in cell division called nondisjunction. This error occurs during the formation of the egg or sperm.

Important points:

• It is not caused by something a parent did or did not do.
• It is typically not inherited.
• The risk increases with advanced maternal age, but it can occur at any age.

There are three main types:

1. Full trisomy 18 (most common): Every cell has an extra chromosome 18.
2. Mosaic trisomy 18: Only some cells have the extra chromosome.
3. Partial trisomy 18: Only part of chromosome 18 is duplicated.

Full trisomy 18 is associated with the most severe outcomes.

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Signs and Symptoms of Trisomy 18

Trisomy 18 affects many parts of the body. Symptoms can vary, but common findings include:

Before Birth (Prenatal Signs)

• Slow growth in the womb (intrauterine growth restriction)
• Heart defects visible on ultrasound
• Abnormal amniotic fluid levels
• Clenched hands with overlapping fingers
• Structural abnormalities seen on fetal imaging

At Birth

• Low birth weight
• Small head (microcephaly)
• Small jaw (micrognathia)
• Low-set ears
• Clenched fists with overlapping fingers
• Rocker-bottom feet
• Weak muscle tone

Internal Complications

• Congenital heart defects (very common)
• Feeding difficulties
• Breathing problems
• Kidney abnormalities
• Severe developmental delays

Because trisomy 18 affects so many systems, babies often have complex medical needs.

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How Is Trisomy 18 Diagnosed?

During Pregnancy

Trisomy 18 can sometimes be suspected during routine prenatal care.

Screening tests may include:

• First-trimester combined screening (blood test + ultrasound)
• Noninvasive prenatal testing (NIPT), which analyzes fetal DNA in the mother's blood
• Second-trimester quad screening

If a screening test suggests an increased risk, diagnostic tests are offered:

• Chorionic villus sampling (CVS)
• Amniocentesis

These tests analyze the baby's chromosomes and can confirm trisomy 18.

If you're noticing concerning symptoms or changes during your pregnancy, Ubie's free AI-powered symptom checker can help you better understand your symptoms and prepare questions for your healthcare provider.

After Birth

If not diagnosed during pregnancy, trisomy 18 may be suspected based on physical features and confirmed with:

• Chromosome analysis (karyotype)
• Genetic testing

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What Is the Prognosis?

This is the hardest part of the discussion.

Trisomy 18 is considered a life-limiting condition.

• Many affected pregnancies end in miscarriage or stillbirth.
• Among babies born alive:
  • About 50% survive beyond the first week.
  • Approximately 5–10% survive beyond the first year.

Some children, especially those with mosaic trisomy 18, may live longer. However, nearly all survivors have:

• Significant intellectual disability
• Severe developmental delays
• Ongoing medical complications

It is important to understand that while survival beyond infancy is possible, it is uncommon with full trisomy 18.

This information is not meant to remove hope — but to provide medical clarity so families can make informed decisions.

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What Are the Treatment Options?

There is no cure for trisomy 18. Treatment focuses on supportive care and managing complications.

Care decisions are highly individualized and may include:

Supportive (Comfort-Focused) Care

• Feeding support
• Oxygen therapy
• Pain management
• Palliative care involvement

Medical Interventions

In some cases, families choose more intensive treatment, which may include:

• Surgery for heart defects
• Breathing support
• Tube feeding
• Hospital-based care for infections

The medical community increasingly emphasizes shared decision-making, meaning:

• Parents are fully informed.
• Options are discussed openly.
• Decisions align with family values and the baby's comfort.

There is no single "right" approach. The best plan is the one developed in close discussion with neonatologists, pediatric specialists, and genetic counselors.

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Emotional and Psychological Impact

A trisomy 18 diagnosis can bring:

• Shock
• Grief
• Confusion
• Guilt
• Anger

All of these responses are normal.

Parents often describe feeling torn between hope and realism. It is important to allow space for both.

Support may include:

• Genetic counseling
• Perinatal palliative care teams
• Mental health professionals
• Support groups for families facing similar diagnoses

You do not have to process this alone.

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Medically Approved Next Steps

If trisomy 18 has been suspected or diagnosed, consider these steps:

1. Confirm the Diagnosis

If based on screening alone, discuss diagnostic testing with your doctor.

2. Meet With a Genetic Counselor

They can:

• Explain the type of trisomy 18
• Discuss recurrence risk in future pregnancies
• Help interpret complex medical information

3. Consult Pediatric Specialists

You may meet with:

• Neonatologists
• Pediatric cardiologists
• Maternal-fetal medicine specialists

This helps you understand likely medical needs after birth.

4. Discuss Care Goals Early

Ask your care team:

• What interventions are possible?
• What are the risks and benefits?
• What does comfort-focused care involve?

5. Seek Emotional Support

Professional counseling can be very helpful during decision-making.

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When to Speak to a Doctor Immediately

During pregnancy, seek urgent medical care if you experience:

• Severe abdominal pain
• Heavy vaginal bleeding
• Decreased fetal movement
• Signs of preterm labor

After birth, immediate medical attention is needed for:

• Breathing difficulty
• Poor feeding
• Blue discoloration of skin
• Fever

Any potentially life-threatening or serious symptom requires immediate evaluation by a healthcare professional.

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A Balanced Perspective

Trisomy 18 is a serious and often life-limiting condition. That is the medical reality.

At the same time:

• Every child is unique.
• Some families cherish meaningful time, even if brief.
• Advances in neonatal and supportive care continue to evolve.

The most important step is not to navigate this alone.

If you are facing concerns about trisomy 18, start by gathering accurate information, use Ubie's free AI-powered pregnancy symptom checker to better understand any symptoms you're experiencing, and then speak directly with a qualified medical professional.

Above all, speak to a doctor promptly about any serious or life-threatening concerns. Early medical guidance ensures you receive appropriate testing, clear information, and compassionate support.

You deserve clarity, honesty, and expert care as you move forward.