Trisomy 18, also called Edwards syndrome, occurs in roughly 1 in 5,000 to 1 in 7,000 live births. That makes it the second most common trisomy condition after Down syndrome, but it is still considered rare. The numbers shift significantly depending on whether you count only live births or include pregnancies that end in miscarriage and stillbirth, since the majority of affected pregnancies do not reach full term.
Prevalence at Birth and in Pregnancy
A multi-registry population analysis published through the CDC found an average prevalence of 1.07 per 10,000 live births for trisomy 18. That translates to roughly 1 in every 9,300 babies born alive. However, this figure tells only part of the story. Trisomy 18 is far more common at conception and during early pregnancy than it is at delivery, because most affected pregnancies end before birth.
Research estimates that about 64% of fetuses diagnosed with trisomy 18 at midtrimester amniocentesis will not survive to delivery. In one study tracking pregnancies that continued after a trisomy 18 diagnosis, about 32% of fetuses died before birth. The gap between those numbers reflects differences in when the pregnancies were identified and how long they were followed. The takeaway is that trisomy 18 is substantially more common as a chromosomal finding in pregnancy than live birth statistics suggest. Some estimates place the rate of trisomy 18 at conception as high as 1 in 2,500, with most losses occurring in the first and second trimesters.
How Maternal Age Affects the Risk
Like other trisomy conditions, trisomy 18 becomes more likely as maternal age increases. The risk rises because errors in chromosome separation during egg cell development are more common in older eggs. At age 35, the estimated risk of trisomy 18 is about 1 in 1,111. By age 40, that risk climbs to roughly 1 in 333, a threefold increase over just five years. While trisomy 18 can occur at any maternal age, the majority of cases cluster in pregnancies where the mother is over 35.
More Girls Than Boys Are Born With It
One distinctive pattern with trisomy 18 is that it disproportionately affects females at birth. Roughly 80% of live-born infants with the condition are girls, a ratio of about three or four to one. This isn’t because females are more likely to develop the chromosomal error. When trisomy 18 is diagnosed prenatally, the sex ratio is much closer to even, at about 0.90 males for every female. The imbalance at birth appears to result from male fetuses with trisomy 18 being less likely to survive pregnancy. Something about the combination of male sex and the extra chromosome 18 leads to higher rates of fetal loss after midpregnancy.
Full Trisomy vs. Mosaic and Translocation Forms
Most cases of trisomy 18, roughly 94%, are “full” trisomy, meaning every cell in the body carries three copies of chromosome 18 instead of two. A small percentage of cases are mosaic trisomy 18, where only some cells have the extra chromosome while others are normal. Mosaic cases tend to be less severe, and individuals with this form generally live longer and may have milder physical features. An even smaller fraction result from a translocation, where extra chromosome 18 material attaches to another chromosome. Mosaic and translocation forms together account for only about 5 to 6% of all trisomy 18 cases, which makes them quite rare within an already rare condition.
Prenatal Screening and Detection
Trisomy 18 is now routinely screened for during pregnancy. Non-invasive prenatal testing (NIPT), which analyzes fragments of fetal DNA circulating in the mother’s blood, detects 98 to 99% of trisomy 18 pregnancies. The false positive rate is low, about 0.2%, or 1 in 500 tests. That means a positive NIPT result is a strong signal but not a definitive diagnosis.
Because NIPT is a screening tool rather than a diagnostic one, a positive result is typically followed by amniocentesis or chorionic villus sampling to confirm the finding. These diagnostic tests analyze fetal chromosomes directly and are considered definitive. First-trimester combined screening, which uses blood markers and an ultrasound measurement of fluid at the back of the baby’s neck, can also flag trisomy 18 risk, though with somewhat lower accuracy than NIPT.
What Trisomy 18 Means for Survival
Trisomy 18 carries a very serious prognosis. The extra chromosome disrupts development of nearly every organ system, and heart defects are present in the vast majority of cases. These cardiac problems, combined with issues affecting the lungs, kidneys, and brain, mean that most infants with full trisomy 18 do not survive long after birth. About 50% of babies born alive with the condition survive past the first week, and only about 5 to 10% reach their first birthday.
Children with mosaic trisomy 18 tend to have better outcomes, sometimes significantly so, depending on the proportion of affected cells. Some individuals with mosaic forms have survived into their twenties and thirties, though this remains uncommon. For full trisomy 18, long-term survival past childhood is exceedingly rare. Families facing this diagnosis are often counseled about comfort-focused care, though some centers now offer selective surgical interventions for specific problems like heart defects, which has extended survival in a small number of cases.
Comparing Trisomy 18 to Other Chromosomal Conditions
- Down syndrome (trisomy 21): Occurs in about 1 in 700 live births, making it roughly 13 times more common than trisomy 18 at birth. Most individuals with Down syndrome survive into adulthood.
- Patau syndrome (trisomy 13): Rarer than trisomy 18 at about 1 in 16,000 live births. It carries a similarly severe prognosis, with most affected infants not surviving the first year.
- Turner syndrome (monosomy X): Affects about 1 in 2,500 female births. It is far more survivable, and most individuals live normal lifespans with appropriate medical management.
Trisomy 18 occupies a difficult middle ground: common enough that most obstetricians and genetic counselors encounter it regularly, but rare enough that many families have never heard of it before receiving a prenatal diagnosis. The combination of its relatively low incidence at birth and its severe impact on survival makes it one of the more challenging diagnoses in prenatal medicine.