What is Down Syndrome?

It still may not be fully understood, but here, doctors explain what they know about Down syndrome.

Having a baby with Down Syndrome can be overwhelming, Here are some important things to know about this condition and the road ahead for you and your infant.

Because Down syndrome occurs in 1 out of every 700 babies, odds are good that you've known people with the condition throughout your life. And because they tend to share a variety of physical characteristics (a flattened face, almond-shaped eyes that may slant up, a short neck, small ears, or short stature), you may think you can spot Down syndrome when you see it. But Down syndrome is much more than simply having a certain appearance -- in fact, it is entirely possible to have Down syndrome without having many of those physical traits. "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 Emily Jean Davidson, M.D., clinical director of the Down syndrome program at Boston Children's Hospital. So do you really know what it is or why it happens? Read on.

The Chromosome Connection

Down syndrome is a condition that happens when a baby is conceived with a full or partial extra copy of chromosome 21. Most of us have 46 chromosomes (rodlike bundles of genes) in the nucleus of every cell in our body. 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 the baby ends up with three copies of that chromosome, which is known as a trisomy. About 95 percent of people with Down syndrome have three copies of the 21st chromosome, so the condition is also referred to as trisomy 21. In about 3 percent to 4 percent of children with Down syndrome, there are two full 21st chromosomes and a partial extra chromosome 21 stuck to a different chromosome altogether, the result of a process called translocation. Even more rare is a version of the condition known as mosaic Down syndrome, in which some cells have three copies of chromosome 21 but others have only the usual pair; children with mosaic Down syndrome may have fewer features of the condition because other presence of these cells with the typical number of chromosomes.

What Causes Down Syndrome?

British physician John Langdon Down first described the condition in 1866, although researchers did not begin to understand the chromosomal basis of Down syndrome until 1959. And even though we now understand how an extra 21st chromosome can cause Down syndrome, we don't yet know precisely what causes someone to inherit that third copy. "We know 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.

Growing Up With Down Syndrome

What we do know is that having an extra 21st chromosome can affect the way a baby develops in many ways. The first is developmental: "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, Dr. Davidson notes. "Most children with Down syndrome continue to learn throughout their childhood and as adults, with varying degrees of support."

There are also certain physical issues that are more common in children with Down syndrome. About 50 percent are born with congenital heart defects, most of which can be corrected surgically in the first few months of life. "Down syndrome used to have a 10 percent mortality rate in the first year of life, mostly due to these 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."

Children with Down syndrome are also at a higher risk for hearing loss, vision impairment, thyroid problems, 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." 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 and know which extra tests and screenings your child should receive to track the most likely issues. 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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