T18, or trisomy 18, is a serious chromosomal condition where a baby has three copies of chromosome 18 instead of the usual two. Also called Edwards syndrome, it occurs in roughly 1 in 10,000 live births and causes severe developmental problems that affect nearly every organ system. Most people encounter this term for the first time on a prenatal screening result, and understanding what it means is an important first step.
What Causes Trisomy 18
Every cell in your body normally carries 23 pairs of chromosomes, for a total of 46. In trisomy 18, an error during cell division (called nondisjunction) leaves the embryo with an extra copy of chromosome 18. That additional genetic material disrupts development from very early on, leading to the physical and organ abnormalities associated with the condition. The error is most often maternal in origin, and the likelihood increases with advancing maternal age.
There are three forms. Full trisomy 18 is by far the most common: every cell in the body carries the extra chromosome. About 5% of cases are mosaic trisomy 18, where only some cells have the extra copy. Mosaic cases vary widely in severity depending on how many cells are affected, and some individuals develop more normally than others. Very rarely, a piece of chromosome 18 attaches to a different chromosome (a translocation). In partial trisomy 18, severity depends on how much extra genetic material is present.
How T18 Is Detected During Pregnancy
T18 can show up through screening tests, diagnostic tests, or ultrasound findings. These often happen at different points in pregnancy and serve different purposes.
Prenatal Screening
Non-invasive prenatal testing (NIPT), a blood draw typically offered after 10 weeks of pregnancy, analyzes fragments of fetal DNA circulating in your blood. For trisomy 18, large meta-analyses report sensitivity around 97.7% to 97.9% and specificity of 99.9%. That means the test catches nearly all true cases and produces very few false positives, but it is still a screening tool, not a diagnosis. A positive NIPT result means the probability is high enough that confirmatory testing is recommended.
First-trimester combined screening (a blood test plus a nuchal translucency ultrasound measurement, usually done between 11 and 14 weeks) can also flag an elevated risk for T18, though it is less precise than NIPT.
Diagnostic Confirmation
The only way to definitively confirm trisomy 18 is by analyzing the baby’s chromosomes directly. Two procedures can do this. Chorionic villus sampling (CVS) takes a tiny sample of placental tissue and is typically performed at 10 to 12 weeks. Amniocentesis draws a small amount of amniotic fluid and is usually done between 15 and 18 weeks. Both produce a full chromosome map, called a karyotype, that will show whether the extra chromosome 18 is present and which form of trisomy is involved.
Ultrasound Markers
Sometimes T18 is first suspected not from a blood test but from what a sonographer sees on a routine ultrasound. Common findings include heart defects, clenched fists with overlapping fingers, choroid plexus cysts (small fluid pockets in the brain), club or rocker-bottom feet, an abdominal wall defect called an omphalocele, kidney abnormalities, and intrauterine growth restriction. No single marker confirms trisomy 18 on its own, but the combination of several raises suspicion enough to recommend diagnostic testing.
What Trisomy 18 Means for the Baby
Trisomy 18 affects nearly every part of the body. Heart defects are present in the majority of cases. Brain, kidney, and gastrointestinal abnormalities are also common. Babies with full trisomy 18 are typically very small at birth and face serious, life-limiting medical challenges.
Survival statistics are difficult to summarize neatly because outcomes depend heavily on the form of trisomy, the specific organs affected, and the level of medical intervention a family chooses. In one retrospective review of 37 liveborn infants with complete trisomy 18, babies born through planned delivery for pregnancy interruption survived from minutes to under four hours. Among infants who were discharged home with their families, about two-thirds survived to one year. That contrast reflects the wide range of clinical situations families face and the influence of the care approach chosen.
Mosaic trisomy 18 generally carries a better prognosis than the full form, though outcomes still vary considerably depending on which and how many cells are affected.
Care Options After Diagnosis
A prenatal diagnosis of trisomy 18 leads to several possible paths, and the right choice is deeply personal. Some families choose to continue the pregnancy with a comfort-focused (palliative) plan, prioritizing the baby’s quality of life after birth without pursuing aggressive interventions. Others opt for more active medical treatment, which can include cardiac surgery, respiratory support, and feeding assistance through a surgically placed tube.
The medical community’s approach to trisomy 18 has shifted over the past two decades. It was once standard to offer only comfort care, but more recent evidence shows that some interventions, particularly heart surgery, can meaningfully extend survival and improve quality of life for certain children. Current thinking emphasizes evaluating each child’s individual clinical picture and discussing all options, from comfort care to surgical intervention, so families can make decisions aligned with their child’s best interest.
Some families, depending on gestational age and local laws, may also consider ending the pregnancy after a confirmed diagnosis. Genetic counseling is a key part of this process, helping parents understand the diagnosis, the range of outcomes, and the options available to them.
Risk of Recurrence in Future Pregnancies
For families who have had a pregnancy affected by full trisomy 18, the recurrence risk is 0.5% to 1% in subsequent pregnancies. That is higher than the general population risk, but still relatively low. The picture is different for partial trisomy 18 caused by a translocation. If one parent carries a balanced translocation (meaning their chromosomes are rearranged but no genetic material is missing), the recurrence risk can be as high as 20%. Genetic testing of both parents after a diagnosis can clarify which situation applies and guide counseling for future pregnancies.
Because the frequency of nondisjunction errors rises with maternal age, the overall population prevalence of trisomy 18 has increased in recent decades as the average age of childbearing has gone up. Prenatal screening is available in every subsequent pregnancy regardless of age, and families with a prior T18 diagnosis are typically offered early and thorough testing.