Down syndrome may not be fully understood, but doctors keep learning more about the chromosomal disorder. Here’s what to know about the causes, symptoms, and diagnosis of Down syndrome in babies and children.

By Virginia Sole-Smith and Nicole Harris
Updated September 09, 2020
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Down syndrome affects one out of every 700 babies, according to the Centers for Disease Control and Prevention (CDC), so you’ve probably known people with the condition. But despite the fact that many individuals with Down Syndrome share physical characteristics—including a flattened face, slanted almond-shaped eyes, a short neck, small ears, and short stature—you can’t always recognize it. That’s because Down syndrome has a variety of different physical and intellectual markers that vary between people. Keep reading to learn more about the causes, symptoms, and life expectancy for this chromosomal disorder.

What Causes Down Syndrome?

British physician John Langdon Down first described Down syndrome in 1866, although researchers did not begin to understand the chromosomal basis until 1959. Most people have 46 chromosomes (rod-like bundles of genes) in the nucleus of every cell. Those chromosomes come in pairs, and we inherit 23 of them from our mother and 23 from our father. But sometimes a parent passes along two copies of a specific chromosome, so a baby ends up with three copies of that chromosome, which is known as a trisomy. Down syndrome usually results when a child has extra copies of the chromosome 21.

Though we now understand that an extra copy of chromosome 21 causes Down syndrome, we don't yet know what makes someone inherit that third copy. "Maternal age plays a role but that's not the whole story. When a family asks me why this happened to their child, we just don't understand this as well as we would like," explains Kenneth Rosenbaum, M.D., founder of the division of genetics and metabolism and co-director of the Down Syndrome Clinic at Children's National Medical Center in Washington, D.C. Generally, there’s no way to predict or prevent Down syndrome when trying to conceive.

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Types of Down Syndrome

There are three types of Down syndrome: trisomy 21, translocation Down syndrome, and mosaic Down syndrome.

Trisomy 21: The baby has three copies of chromosome 21 in every cell. This abnormal cell division causes about 95 percent of Down syndrome cases, and it can result from either the sperm cell or the egg cell.

Translocation Down syndrome: In about 3 percent to 4 percent of children with Down syndrome, the baby has two copies of chromosome 21 like usual. But they also have a full or partial extra chromosome 21, which is attached to a different chromosome altogether—usually chromosome 14, according to the National Down Syndrome Society.  This is the result of a process called translocation, and it can cause the typical Down syndrome characteristics.

Mosaic Down Syndrome: In mosaic Down syndrome, some cells have three copies of chromosome 21, but others have only the usual pair. This is the rarest form of Down syndrome, and it accounts for only 1-2 percent of cases. Children with mosaic Down syndrome may have fewer features of the condition because of the presence of cells with the typical number of chromosomes.

Down Syndrome Symptoms

Having an extra copy of chromosome 21 can affect a baby in many ways, both intellectually and physically. Here’s what to know about Down syndrome characteristics and symptoms.

Intellectual Characteristics of Down Syndrome

“Down syndrome is the most common known cause of intellectual disability," Dr. Rosenbaum says. "When it comes to speech and learning, the quality of the skills that children with Down syndrome acquire is different and they tend to acquire new skills more slowly. But we see a huge range in terms of what they are capable of achieving—some will always need special education, while others can be in mainstream classrooms," he explains.

One of the biggest misconceptions about Down syndrome is that children will reach a point where they plateau or stop learning, notes Emily Jean Davidson, M.D., clinical director of the Down syndrome program at Boston Children's Hospital. "Most children with Down syndrome continue to learn throughout their childhood and as adults, with varying degrees of support," she adds.

Physical Characteristics of Down Syndrome

You may notice that children with Down syndrome share some physical characteristics.  "All the physical characteristics mean is that children with Down syndrome look a little bit like other children with Down syndrome, while also looking like their own family members, of course," says Dr. Davidson. 

Although every person with Down syndrome will have different physical symptoms, here are some of the most common ones:

  • Flattened face
  • Almond-shaped eyes with an upward slant
  • Short stature 
  • Low muscle tone
  • Small neck, ears, hands, and/or feet
  • Protruding tongue 
  • Palmar crease (deep crease across the palm)
  • Small pinky fingers that sometimes curve
  • Tiny white spots on iris of the eye

About 50 percent of children with Down syndrome are born with congenital heart defects, most of which can be corrected surgically in the first few months of life. They’re also at a higher risk for hearing loss, vision impairment, thyroid problems, immune disorders, and sleep apnea, among other issues. "Almost every system of the body can have an issue as a result of the extra genetic information from the extra 21st chromosome," Dr. Davidson says. "But that doesn't mean that everyone gets everything." 

Diagnosing Down Syndrome in Babies

Two tests can help diagnose Down syndrome: screening tests and diagnostic tests.

Screening tests: Screenings often involve a blood test and an ultrasound. Blood tests measure substances in the mother’s blood, while ultrasounds scan for excess fluid behind the fetus’s neck. The presence of excess fluid could indicate Down syndrome. Screenings won’t harm the baby but they aren’t always accurate in detecting abnormalities. Instead, they give a probability that your child could have Down syndrome.

Diagnostic tests: While diagnostic tests are more accurate at predicting Down syndrome, they come with a very low risk of miscarriage. Doctors may use chorionic villus sampling (CVS) to analyze fetal chromosomes from placenta cells between 10 and 13 weeks of pregnancy. They can also examine the amniotic fluid with an amniocentesis after 15 weeks of pregnancy. 

Down syndrome can be diagnosed after the baby is born based on physical appearance and a chromosomal karyotype test.

Down Syndrome Life Expectancy 

"Down syndrome used to have a 10 percent mortality rate in the first year of life, mostly due to heart problems," Dr. Rosenbaum says. "That's not true today. Now we have babies going home within a week of their heart surgery and doing very well." While life expectancy varies for each individual, many people with Down syndrome have predicted life spans that exceed 60 years.

Your pediatrician or a Down syndrome specialist—such as a doctor who is board-certified in clinical genetics, developmental pediatrics, or neurodevelopmental disabilities—will tell you what red flags to look for. They'll also advise on extra tests and screenings your child should receive to track the most likely issues.

Down Syndrome Treatment

Down syndrome treatment simply involves catering to your child's intellectual and physical needs. Depending on the child, this could include seeing specialized doctors for health complications, enrolling in special needs classes, and taking occupational, speech, or physical therapy. Notes Dr. Rosenbaum: "Most children are not any sicker than their brothers and sisters and have an excellent quality of life."

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