What is Trisomy 18?

Trisomy 18 (also known as Edwards Syndrome) is a chromosomal abnormality in which a person inherits three copies of chromosome 18 in every cell of the body, rather than the normal two. This extra bit of genetic material on chromosome 18 disturbs normal development.

What Causes Trisomy 18?

When the sperm and ova meet during conception, 23 chromosomes from the mother and 23 chromosomes from the father combine to form a baby with 46 chromosomes per cell. A trisomy happens when an additional chromosome is added at the time of conception. For those with Trisomy 18, an extra chromosome is added to the 18th chromosomal pair.

A trisomy is never, ever the parents' fault, nor is it caused by anything that happened during pregnancy.

How Common Is Trisomy 18?

In live births (rather than stillbirths), Trisomy 18 is the second most common autosomal trisomy, following only Down Syndrome. Trisomy 18 occurs in approximately 1 in every 6,000-8,000 newborns. 80% of newborns with Trisomy 18 are female. During first trimester prenatal screening, the incidence of Trisomy 18 is 1:400; however, due to the high amounts of spontaneous losses, the birth prevalence is 1:6,500.

What Are The Types of Trisomy 18?

  • Full Trisomy 18 - the most common type - 95% of all Trisomy 18 cases - is a full Trisomy, meaning that the extra chromosome exists in every cell in the body. Full Trisomy 18 is not hereditary and is not the result of actions before or during pregnancy.
  • Partial Trisomy 18 - a rare form of Trisomy 18 that occurs when a fragment of an extra chromosome exists and may be hereditary. People who have partial Trisomy 18 have two copies of chromosome 18 in addition to a small bit of extra material - rather than a full extra chromosome - on chromosome 18.
  • Mosaic Trisomy 18 - a rare form of Trisomy 18 that occurs when the extra chromosome is present in some - but not all - cells within the body. Mosaic Trisomy 18 is not inherited, but a random occurrence that happens during cell division.

Epidemiology of Trisomy 18:

Age: Trisomy 18 is detectible during newborn as well as prenatal periods.

Race: Trisomy 18 has no racial proclivity.

Sex: 80% of Trisomy 18 cases occur in females.

Morbidity/Mortality of Trisomy 18:

As many as 95% of conceptions with Trisomy 18 pass away as fetuses or embryos. 5-10% of children affected with Trisomy 18 live beyond the first year of life.

For livebirths with Trisomy 18, the median survival is 14.5 days. The extremely high mortality rate is due to renal and cardiac malformations, sepsis, apnea and feeding difficulties. 50% of infants may survive into the teen years, but will do so with severe medical problems and developmental delays.

Long-term survival up to 27 years of age has been reported.

Prenatal Signs of Trisomy 18:

  • Single umbilical artery
  • Weak fetal activity
  • Fetal distress
  • Intrauterine growth restriction
  • Excess amniotic fluid or inadequate amniotic fluid
  • Small placenta

Physical Signs of Trisomy 18:

The extra genetic material in individuals with Trisomy 18 can cause a spectrum of problems with varying severity, ranging from extremely affected to mildly affected, meaning that there is no specific definition as to how Trisomy 18 will affect each individual.

Organ systems commonly affected include:

Cardiac - 90% of infants with Trisomy 18 have cardiac malformations. The most common cardiac malformations are ventricular septal defects (holes in the septum, or wall, separating the left and right ventricles of the heart) along with pulmonary and aortic valve defects. Other malformations include:

  • patent ductus arteriosus - failure of the ductus arteriosus, the blood vessel that allows blood to go around the baby's lungs before birth, to close after birth
  • overriding aorta - the aorta is shifted over the right ventricle and ventricular septal defect, instead of coming out only from the left ventricle
  • atrial septal defects - holes in the wall separating the left and right upper chambers (atria) of the heart
  • coarctation of the aorta - narrowing of part of the aorta
  • tetralogy of Fallot - a combination of 4 related heart defects - ventricular septal defect, pulmonary stenosis (narrowing of the pulmonary valve), right ventricular hypertrophy (thickening of the right ventricular muscle wall), and overriding aorta
  • transposition of the great arteries - the two main arteries leaving the heart are reversed, changing the way blood circulates through the body
  • hypoplastic left heart syndrome - the left side of the heart is severely underdeveloped and can't effectively pump blood out to the body


  • abnormal lobation of the lungs - the lungs are irregular in shape
  • pulmonary hypoplasia - the lungs are underdeveloped


  • microcephaly - head size is significantly smaller than it should be for an infant of the same gestational age
  • wide fontanels - larger than expected fontanels (soft spots) on the baby's head, where the skull bones come together
  • long skull


  • delayed motor development and mental retardation in 100% of cases
  • poor muscle tone
  • apnea - the infant stops breathing at certain intervals
  • seizures
  • microcephaly
  • Arnold-Chiari malformation
  • hydrocephalus - excessive build-up of cerebrospinal fluid in the brain
  • frontal lobe defect
  • arachnoid cysts - sacs filled with cerebrospinal fluid, situated between the brain (or spinal cord) and the arachnoid membrane (one of the three membranes covering the brain/spinal cord)
  • myelomeningocele


  • adrenal hypoplasia - underdeveloped adrenal glands
  • thyroid hypoplasia - underdeveloped thyroid gland
  • thymic hypoplasia - underdeveloped thymus gland


  • pyloric stenosis - narrowing of the opening from the stomach into the small intestine
  • exstrophy of Cloaca - the bladder and portion of the intestines are exposed outside the abdomen; the intestine is short and the anus may not be open
  • imperforate anus - the opening to the anus is missing or blocked
  • malpositioned anus
  • pilonadal sinus - a cavity below the skin surface near the crease of the buttocks that connects to the surface with one or more small openings
  • hernias - umbilical, inguinal, or diaphragmatic - holes in the mesentery or membrane that holds the organs in place
  • omphalocele - the intestines or other abdominal organs protrude through the belly button
  • ileal atresia - an obstruction in the ileum, the final and longest section of the small intestine, that connects to the large intestine
  • esophageal atresia - an obstruction in the esophagus
  • malrotation of the intestine - the intestines do not rotate completely or correctly during gestation, causing intestinal blockages or twisting
  • Meckel diverticulum - pouch on the wall of the lower part of the intestine that may contain tissue identical to the stomach or pancreas
  • diastasis recti - separation between the left and right side of the abdominal muscle that covers the front belly area
  • absent gallbladder
  • absent appendix
  • accessory spleens


  • double ureters - the infant has two ureters draining the kidney, instead of one
  • hydroureters - swelling in the ureters
  • hydronephrosis - swelling in the kidneys
  • horseshoe kidneys - kidneys are fused together at the base
  • micromulticystic kidneys - tiny cysts in the kidneys
  • unilateral renal agenesis - the infant only has one kidney instead of two
  • Females may have hypoplasia (underdevelopment) of labia, ovaries, or clitoris.
  • Males may have micropenis (unusually small penis), hypospadias (urethral opening on the underside of the penis), or cryptorchism (undescended testes).

How Is Trisomy 18 Diagnosed?

There are two types of prenatal testing available for Trisomy 18. These include screening and diagnostic exams.

1) Screening for Trisomy 18: A screening test may indicate a risk that the fetus has Trisomy 18 by taking all results from the same test and comparing results to individual results. A screening is NOT the same as a diagnosis, it merely indicates that the risk for Trisomy 18 is higher than the average.

  • AFP (also known as triple screen, quad screen, maternal serum screening) is a blood test performed between week 15 and 17 of pregnancy that detects levels of AFP, hCG and Estriol. An elevated AFP may indicate a risk for Trisomy 18.
  • Ultrasonography - In the case that the AFP returns a risk for Trisomy 18, a level II ultrasound may be ordered. These more detailed ultrasounds performed by a perinatologist or high risk OB will look for markers of a chromosomal abnormality. These markers include soft markers - characteristics of babies with Trisomy 18 - and structural markers - abnormalities in the bone and organ formation. Several markers for Trisomy 18 include: rocker bottom feet, delayed growth, clenched fists, heart defects, kidney problems and esophageal atresia. Even if many markers for Trisomy 18 are discovered on an ultrasound, this is not a diagnosis of Trisomy 18.

2) Diagnostic Tests for Trisomy 18: Diagnostic tests are able to check the genetic material of the fetus to ascertain if Trisomy 18 is present. These tests are able to diagnose Trisomy 18 and include:

  • Chorionic Villi Sampling (CVS) - a diagnostic test performed between weeks 10 and 12 gestation that involves taking a piece of the chorionic villi and analyzing the cells. As a CVS generally has the same genetic and biochemical material as the fetus, chromosomal issues may be diagnosed.
  • Amniocentesis - a diagnostic test performed typically between weeks 15 and 18 of pregnancy in which a small amount of amniotic fluid (which contains fetal tissues) is removed from the amniotic sac surrounding the fetus. The amniotic fluid is then tested for genetic abnormalities.

Decisions After A Prenatal Diagnosis of Trisomy 18:

Finding out that your child has a diagnosis of Trisomy 18 is one of the most difficult experiences, and nothing can be done to make it any easier. If your child has been diagnosed with Trisomy 18, be certain to discuss this diagnosis with your doctor and a genetic counselor.

  • Ask questions, take notes, and ask for the doctors to explain everything.
  • Seek out doctors that you trust and who will advocate for you - no matter what decision you make.
  • Learn as much as you can about Trisomy 18
  • Take all of the time you need before making any decisions.
  • Remember: it's okay to grieve the child you'd pictured.
  • Keep in mind that every child, every pregnancy is different, and Trisomy 18 can have a spectrum of symptoms.

If A Trisomy 18 Diagnosis Is Made After Birth:

Medical care for babies with Trisomy 18 is supportive and aimed at treating and preventing infections, encouraging feeding as well as treatment for cardiac and other structural abnormalities.

During the neonatal period, issues of diagnosis (especially if it was unexpected) and survival are paramount and parents should be educated about Trisomy 18, the implications of the condition as well as the multitude of outcomes.

Parents should be directed to support services within the hospital and the community as the presence of a disabled child in a family is a tremendous source of anxiety and stress.

Parents should expect the roller-coaster of grief and grieving including reactive grief related to chronic illness and preparatory grief associated with losing a child.

Parents should be advised that they did nothing to cause this condition in their child and that the risk of recurrence in subsequent pregnancies is less than 1% (in cases with full trisomy).

Related Resource Pages on Band Back Together:

High Risk Pregnancy

Special Needs Parenting

Chiari Malformation


Down Syndrome

Spina Bifida

Congenital Heart Defects


Baby Loss

Child Loss



What To Say To Someone Who Has Lost A Child


Additional Resources:

Trisomy 18 Foundation - the mission of this foundation is to encourage the search for treatments and preventions of Trisomy 18, to educate and support medical professionals, and to create a caring worldwide community for affected families.

Support Organization for Trisomy 18, 13 and Related Disorders (SOFT) - a nonprofit volunteer organization offering support for parents who have had or are expecting a child with a chromosome disorder (especially Trisomy 18 and Trisomy 13), and education to families and professionals interested in the care of these children.

Hope for Trisomy 13 & 18 - non-profit that is devoted to funding research for Trisomy 13 & 18 and related chromosome disorders, develop programs that educate parents of children with Trisomy about options available to their child and teach them how to advocate for their child to receive all available treatment, and to create materials, literature, workshops, seminars, and conferences to educate the medical health professionals, the parents, and the public about Trisomy 13 and 18, so they have accurate and current information and resources available to them.