Trisomy 18, also called Edwards syndrome, is a medical condition caused by the presence of a third copy of chromosome 18. When an extra chromosome is present, extra genetic instructions are present, which affects the baby’s development during pregnancy and after birth. 

Approximately 1 in 5,000 live-born babies is born with trisomy 18. However, more babies with trisomy 18 are conceived than will survive to term, and as many as 90% of pregnancies with trisomy 18 will result in miscarriage or stillbirth due to the complexity of developmental issues caused by the extra chromosome 18. Babies with trisomy 18 tend to have physical differences (also called “birth defects”) that may be seen either on ultrasounds during pregnancy or at birth. For example, heart and kidney defects and poor growth or small size are common features. 

What are the features of trisomy 18?

Trisomy 18 causes a range of physical and cognitive differences. Not every baby with trisomy 18 will have all of these findings, but common features include: 

  • Small size, including small head size and low birth weight
  • Heart defects
  • Kidney abnormalities
  • Low-set ears
  • Triangular face with a narrow mouth and small chin
  • Clenched or overlapping fingers
  • Clubfeet
  • Weak muscle tone (hypotonia)
  • Omphalocele (opening in the abdominal wall)
  • Brain and spinal cord anomalies
  • Intellectual disability 

Advances in modern medicine mean that resources and interventions, such as cardiac surgery or a feeding tube, can help babies with trisomy 18 survive after birth. However, even with advanced care, most babies born with trisomy 18 do not live past 1 year of age due to their physical and developmental challenges. Children with trisomy 18 who live past infancy have significant intellectual disability and require specialized care throughout their life. While physical and intellectual disabilities are expected for children with trisomy 18, it’s important to keep in mind that every individual with trisomy 18 is unique, and no two experiences with this condition are identical.  

What causes trisomy 18? 

The majority of cases of trisomy 18 occur by chance, meaning the condition does not usually run in families. Diet and lifestyle cannot cause or prevent trisomy 18, and a person of any age can have a pregnancy affected by trisomy 18. However, the likelihood of trisomy 18 in pregnancy increases gradually as the age of the genetic mother increases. 

Most of the time, trisomy 18 is caused by having three copies of chromosome 18 in each cell of the body rather than the two copies seen in most people. Humans typically have 23 pairs of chromosomes – 46 chromosomes in total. For each pair of chromosomes, one copy typically comes from the genetic mother’s egg cell, and the other comes from the genetic father’s sperm cell. In trisomy 18, either the egg or the sperm cell contributes an extra copy of chromosome 18, so the baby receives three total copies rather than two. This extra chromosome is usually a chance finding, meaning that nothing the parents did, or didn’t do, could have caused the extra chromosome in their baby. 

Less commonly, in approximately 5% of cases, the extra copy of chromosome 18 may be present in only some of the cells in the body, rather than in every cell. This is called mosaic trisomy 18. The severity of symptoms in mosaic trisomy 18 depends on the proportion and distribution of the cells with the extra chromosome 18. 

Rarely, trisomy 18 may be due to a chromosome rearrangement known as a translocation. A translocation means that all or part of a chromosome is attached (translocated) to another chromosome. If a parent “carries” a translocation involving chromosome 18 (meaning, their chromosomes are rearranged but still “balanced” with respect to the total amount of genetic information”), they could pass on extra chromosome 18 material to their offspring. In these cases, a baby might inherit a translocated piece of chromosome 18 in addition to the two normal copies of chromosome 18 – resulting in three total copies of chromosome 18.

How is trisomy 18 detected? 

Trisomy 18 is diagnosed by examining an individual's number and arrangement of chromosomes. Before birth, trisomy 18 can be diagnosed via prenatal diagnostic procedures such as chorionic villus sampling (CVS) or amniocentesis. 

CVS is done in the first trimester and involves taking a sample (biopsy) of tissue from the placenta using either a catheter through the cervix (transcervical CVS) or a slender needle through the abdomen (transabdominal CVS). This tissue can then be analyzed to look at the placenta’s chromosomes through a test called a karyotype, also referred to as chromosome analysis. Amniocentesis is a procedure typically done in the second trimester, during which a small amount of amniotic fluid is drawn from the amniotic sac using a long, slender needle. A karyotype can be performed using the fetal cells in the amniotic fluid to diagnose chromosome conditions such as trisomy 18. Both CVS and amniocentesis are considered invasive procedures and are associated with pregnancy risks, including a slight risk of pregnancy loss. Research on the outcomes of these procedures over the past several decades suggests that for the average pregnant person, there is approximately a 1 in 300 to 1 in 1,000 chance for miscarriage following CVS or amniocentesis.   

In addition to karyotype, another type of genetic analysis called chromosomal microarray analysis, or CMA, can be performed to look for smaller chromosomal segments that may be extra or missing. These are called microduplications and microdeletions, respectively. Chromosomal microarray analysis can be helpful for detecting cases of partial trisomy 18, where there is an extra piece of only part of chromosome 18 rather than an extra entire chromosome 18. 

After birth, karyotype or chromosomal microarray analysis can be performed on a simple blood sample to diagnose trisomy 18. 

Chromosomal variation causing trisomy 18

How is trisomy 18 treated?

To support babies with trisomy 18, a large team of specialized medical providers is necessary. This team often includes a neonatologist, neurologist, cardiologist, nephrologist, geneticist, surgeon, social worker, genetic counselor, and more. Unfortunately, there is no cure for trisomy 18, and care after birth focuses on stabilization and survival. Some families may choose to proceed with palliative or hospice care to minimize their baby’s pain and maximize their quality of life for whatever time they have together. In other cases, intervention may take the form of multiple surgeries – such as cardiac surgeries to repair congenital heart defects, or craniofacial surgery to repair a cleft lip and/or palate. Parents work closely with the healthcare team to find options that are right for their family, and social support from loved ones is crucial during this time. Although they will face many challenges, each individual with trisomy 18 is unique, and their diagnosis alone cannot predict the exact course of their life. 

Where can I find support or learn more about trisomy 18? 

Trisomy 18 is a complex condition that requires specialized care, and the experience of facing a trisomy 18 diagnosis before or after birth can be an emotionally trying time. To learn more about trisomy 18 and read about families’ journeys with this condition, visit https://trisomy18.org/

- Authored by Kelly Miller, MS, LCGC | Genetic Counselor

 

Please note: JunoDx.com and the materials and information it contains are not intended to, and do not constitute medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.

November 02, 2022 — Stephanie McClintock