Trisomy 18, also called Edwards syndrome, is a genetic condition in which a baby has three copies of chromosome 18 instead of the usual two. It occurs in roughly 1 in 5,000 pregnancies and causes serious health problems affecting the heart, kidneys, and other organs. Most affected pregnancies end in miscarriage or stillbirth, and among babies born alive, the median survival is about 8 days.
How the Extra Chromosome Occurs
Human cells normally contain 23 pairs of chromosomes, for a total of 46. In trisomy 18, something goes wrong during egg or sperm formation: the pair of chromosome 18s fails to separate properly, a process called nondisjunction. The result is a reproductive cell carrying two copies of chromosome 18 instead of one. After fertilization, the embryo ends up with three. The extra copy most often comes from the mother’s egg cell, and the error typically happens during the second stage of cell division.
There are three forms of the condition, and the type matters for prognosis:
- Full trisomy 18 is by far the most common. Every cell in the body carries the extra chromosome. This form causes the most severe effects.
- Mosaic trisomy 18 means some cells have three copies of chromosome 18 while others have the normal two. Because not every cell is affected, symptoms can be milder and more variable.
- Partial trisomy 18 occurs when only a segment of the extra chromosome is present, usually because a parent carries a rearrangement in their own chromosomes. The severity depends on which portion of chromosome 18 is duplicated.
Physical Features and Health Problems
Trisomy 18 affects nearly every organ system. Babies are typically born small for their gestational age and may have a distinctive appearance: a small head, clenched fists with overlapping fingers, low-set ears, and feet with a rounded bottom (sometimes called rocker-bottom feet). These features often prompt doctors to suspect the diagnosis at birth if it wasn’t identified prenatally.
Heart defects are the most significant medical concern. Between 80% and 100% of babies with trisomy 18 have a structural heart problem. The most frequent is a ventricular septal defect, a hole between the heart’s lower chambers, found in roughly 59% of cases. Other common cardiac issues include holes between the upper chambers, abnormal heart valves, and a blood vessel that fails to close after birth.
Kidney problems are also very common. About two-thirds of affected babies have a horseshoe kidney, where the two kidneys are fused together at the base. Feeding difficulties, breathing problems, and slow growth are nearly universal. Many infants have trouble swallowing and may need tube feeding. Seizures, skeletal abnormalities, and gastrointestinal malformations also occur frequently.
Risk Factors
Maternal age is the strongest known risk factor. The likelihood of trisomy 18 rises sharply as a woman gets older: at age 35, the rate is roughly 1.9 per 1,000 pregnancies, climbing to about 5.1 per 1,000 at age 40 and 37 per 1,000 at age 45. For each additional year of maternal age, the odds increase by approximately 18%. That said, because younger women have far more pregnancies overall, babies with trisomy 18 are born to mothers of all ages.
Trisomy 18 is not caused by anything a parent did or didn’t do during pregnancy. In the vast majority of cases it is a random error in cell division. The exception is partial trisomy 18, which can run in families when a parent carries a chromosomal rearrangement without knowing it.
Prenatal Screening and Diagnosis
Most cases of trisomy 18 are now detected before birth. Routine ultrasound may reveal growth restriction, heart defects, or other features that raise suspicion. Blood-based screening tests, including the cell-free DNA test (often called NIPT), can flag the condition with high accuracy. For trisomy 18 specifically, NIPT has a sensitivity around 93% and specificity near 100%, meaning it catches most cases and rarely gives a false alarm.
However, NIPT is a screening test, not a definitive answer. Both false positives and false negatives can happen. If NIPT or another screening test suggests trisomy 18, doctors recommend a diagnostic procedure like amniocentesis or chorionic villus sampling. These tests analyze fetal chromosomes directly and provide a definitive diagnosis. Many families receive the diagnosis during the second trimester, which gives them time to prepare, ask questions, and make decisions about the pregnancy and delivery.
Survival and Prognosis
The outlook for full trisomy 18 is serious. Many affected pregnancies end in miscarriage or stillbirth. Among babies born alive, the median survival is 8 days. About 13.4% survive to their first birthday, and roughly 12.3% reach age 5, according to a multi-state population study covering U.S. births from 1999 to 2007. The small drop between the one-year and five-year figures reflects that children who make it through the first year have already passed the highest-risk period.
Mosaic and partial forms generally carry a better prognosis, though outcomes vary widely depending on how many cells are affected or which chromosome segment is duplicated. Some children with mosaic trisomy 18 survive into their teens or beyond, though intellectual disability and ongoing health needs are typical.
Medical Care and Decision-Making
Caring for a baby with trisomy 18 involves difficult choices. Historically, most newborns with the condition were not offered intensive medical treatment or surgery because of the severity of the diagnosis and concerns about quality of life. Many families chose comfort-focused (palliative) care, prioritizing the baby’s comfort over aggressive intervention.
That approach is shifting. As some children with trisomy 18 have survived longer than expected, and as societal attitudes toward disability have evolved, more families are asking about surgical repair of heart defects and other interventions. Some medical centers have responded by developing multidisciplinary guidelines that weigh the clinical realities against each family’s goals. These conversations typically involve neonatologists, cardiologists, surgeons, palliative care specialists, and ethics teams working together with the family.
There is no single right answer. Some families pursue cardiac surgery and intensive support, while others focus on maximizing comfort and time together. The decision depends on the specific combination of health problems, the type of trisomy 18, and what the family values most. What has changed is that families are increasingly given a choice rather than a single recommendation.