Doctors understand the chromosomal abnormalities behind Down syndrome, but they still can’t pinpoint exactly why it occurs. Here’s what experts know about a baby’s chances of having Down syndrome.

By Virginia Sole-Smith and Nicole Harris
Updated September 09, 2020
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Characterized by an array of physical and intellectual symptoms, Down syndrome affects one in 700 babies today, according to the Centers for Disease Control and Prevention (CDC). And though researchers have pinpointed the chromosomal abnormalities that cause Down syndrome, they still don’t know much about why it happens.

"For the most part, we simply don't understand (Down syndrome causes) as well as we would like," says 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. This can make it difficult to understand whether you’re at risk for having a baby with Down syndrome. Keep reading to learn more about the causes of Down syndrome and what factors may increase your chances.

Causes of Down Syndrome 

To understand your baby’s chances of Down syndrome, it helps to know what causes the condition in the first place. Most people are born with 46 chromosomes (clusters of genes) in the nucleus of every cell. The chromosomes form in pairs with 23 from the mother and 23 from the father. Down syndrome happens when a baby is conceived with a full or partial extra copy of one of these chromosomes—specifically chromosome 21.

Experts have pinpointed three different types of Down syndrome: trisomy 21, mosaic Down syndrome, and translocation Down syndrome.

Trisomy 21: Most people have two copies of chromosome 21 in each cell, but those with trisomy 21 have three copies. About 95 percent of Down syndrome cases are trisomy 21.

Mosaic Down Syndrome: While some cells have two copies of chromosome 21 like usual, others have an additional third copy. Mosaic Down syndrome occurs in 1-2 percent of cases, and affected people may have fewer physical and intellectual characteristics of the condition.

Translocation Down syndrome: There are two full copies of chromosome 21 in the cell, as well as a partial extra chromosome 21 stuck to an entirely different chromosome. This is the result of a process called translocation, and it occurs in 3-4 percent of Down syndrome cases.

What Factors Affect Your Chances of Down Syndrome?

Although Down syndrome is a genetic condition, you most likely will not have any family history of it. "In most cases, the extra chromosome seems to happen by chance," says Emily Jean Davidson, M.D., clinical director of the Down Syndrome Program at Boston Children's Hospital. That said, some factors might increase your chances of having a baby with Down syndrome. Here’s what to know about them.

Maternal Age: Down syndrome can occur at any maternal age, but the possibility increases as a woman gets older. A 25-year-old woman has a one in 1,200 chance of having a baby with Down syndrome. By 35 years of age, the risk increases to one in 350—and it becomes one in 100 by age 40. The chances of Down syndrome further increase to one in 30 by age 45, according to the National Down Syndrome Society. If you're over 35 and thinking about getting pregnant, you and your partner may want to undergo genetic counseling to more precisely pinpoint your risk. 

Maternal Folate Metabolization: "There have been theories about whether it's due to how well the mother metabolizes folate, but there are just as many studies saying no as studies saying yes," Dr. Rosenbaum.

Genetics: Two types of Down syndrome—Trisomy 21 and mosaic Down syndrome—don’t have any heredity component. However, according to the National Down Syndrome Society, translocation Down syndrome is passed from parent to child in one-third of cases. This accounts for 1 percent of all individuals diagnosed with Down syndrome. 

If your baby has a translocation, physicians will suggest checking both parents' chromosomes to see if the translocation arose in the baby (most commonly the case) or if either parent is an unaffected carrier. If so, genetic testing can pinpoint the cause and determine your individual chance of recurrence. Generally, translocation Down syndrome has a 3 percent chance of occurring again if the father carries the translocated chromosome, and a 10-15 percent of recurrence if the mother does, according to the National Down Syndrome Society.

History: If you have one child with Down syndrome, (trisomy 21 or translocation), your chance of having a second child with the condition is about 1 percent. 

No matter your risk, remember that Down syndrome occurs before conception. "Nothing you do during your pregnancy will increase your risk or reverse what has already happened," Dr. Rosenbaum says. "All you can do while pregnant is take the best possible care of yourself and your baby by eating well, taking prenatal vitamins, and following other common sense advice." 

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