What Are Trisomy 13 and Trisomy 18?

Every cell in the human body typically contains 23 pairs of chromosomes, for a total of 46. Trisomy means three copies of a chromosome instead of two.

Trisomy 18 (Edwards syndrome) occurs when there are three copies of chromosome 18. It is the second most common trisomy after Down syndrome, affecting approximately 1 in 5,000 live births.

Trisomy 13 (Patau syndrome) occurs when there are three copies of chromosome 13. It is less common, affecting approximately 1 in 10,000 to 16,000 live births.

Neither condition is caused by anything a parent did or did not do. They result from a random error in cell division, and the risk increases slightly with maternal age.

How They Are Detected

Screening

NIPT (cell-free DNA screening) can flag an elevated risk for both Trisomy 13 and Trisomy 18 from a simple maternal blood draw starting at 10 weeks. First-trimester combined screening (blood work plus nuchal translucency measurement) can also indicate elevated risk.

It is important to understand that a positive screening result is not a diagnosis. NIPT has a higher false-positive rate for Trisomy 13 and 18 than it does for Trisomy 21. A positive screen should always be followed by diagnostic testing.

Diagnostic Confirmation

CVS (10 to 13 weeks) or amniocentesis (15 to 20 weeks) provides definitive chromosomal analysis. Only diagnostic testing can confirm the presence of a full trisomy, a partial trisomy, or mosaicism (where some cells carry the extra chromosome and others do not). The distinction matters, as mosaic forms can have different outcomes.

Ultrasound Findings

Both conditions are often associated with structural differences visible on ultrasound, including heart defects, brain abnormalities, limb differences, and growth restriction. Ultrasound findings in combination with diagnostic testing give the most complete clinical picture.

Medical Realities

Being honest about these conditions is important, and honesty includes acknowledging both the severity and the variation.

Trisomy 18

The medical challenges associated with full Trisomy 18 are significant. Nearly all affected babies have congenital heart defects. Other common findings include kidney malformations, esophageal or intestinal abnormalities, and growth restriction.

Approximately 50% of babies with Trisomy 18 are stillborn. Among those born alive, about 40% survive to one month, and 5% to 10% survive to one year. A small number of individuals, particularly those with mosaic Trisomy 18, live longer, sometimes into their teens or beyond. These longer-term outcomes are becoming more visible as medical care and family advocacy evolve.

Trisomy 13

Full Trisomy 13 presents similarly serious medical challenges. Common findings include heart defects, brain malformations (particularly holoprosencephaly), cleft lip or palate, and eye abnormalities.

Survival statistics are similar to Trisomy 18, with most affected infants not surviving beyond the first weeks or months. Again, mosaic forms may carry different outcomes.

These are difficult numbers to read. They also do not capture the love, connection, and meaning that families experience with their children regardless of how much time they share.

Care Options: A Spectrum of Choices

Families who receive a Trisomy 13 or Trisomy 18 diagnosis face deeply personal decisions, and there is no single right path. The choices include:

Continuing the Pregnancy with Full Medical Intervention

Some families choose to pursue all available medical treatments, including cardiac surgery and other interventions, to extend their child's life as much as possible. A growing number of medical centers are offering more interventional approaches for these conditions, and outcomes data continues to evolve.

Continuing the Pregnancy with Comfort-Focused Care

Perinatal palliative care is a specialized approach that focuses on comfort and quality of life rather than curative treatment. A palliative care team works with you to create a birth plan centered on your values, ensuring your baby is comfortable and that your time together is meaningful.

Perinatal palliative care can include:

  • Birth planning that reflects your wishes (skin-to-skin, photos, family time)
  • Pain management for your baby
  • Emotional and spiritual support for your family
  • Memory-making support (handprints, professional photography, naming ceremonies)
  • Bereavement care and follow-up

Ending the Pregnancy

Some families choose to end the pregnancy after receiving a Trisomy 13 or 18 diagnosis. This decision is never made lightly, and it deserves the same compassion and respect as any other path. Access to this option varies by state, and your care team or a genetic counselor can provide information specific to your situation.

Support for Families

Regardless of the path you choose, you do not have to walk it alone.

  • Genetic counselors can help you understand the diagnosis, your options, and what to expect
  • Social workers at your hospital or clinic can connect you with practical and emotional support
  • Organizations like SOFT (Support Organization for Trisomy 18, 13, and Related Disorders) offer community, resources, and connection with other families
  • Perinatal loss support groups provide a space to grieve if your journey includes loss

Your care team should be a source of information and support without judgment. If you feel pressured in any direction, you have every right to seek providers who honor your autonomy.

Finding Compassionate Care on BAABY

If you are facing a Trisomy 13 or Trisomy 18 diagnosis, the providers around you matter deeply. BAABY's provider directory can help you search for OB-GYNs and maternal-fetal medicine specialists in your area who can guide you through this experience with both expertise and compassion.