Down syndrome happens when a baby is born with an extra copy of chromosome 21. Most people have 46 chromosomes in each cell, arranged in 23 pairs. A baby with Down syndrome has 47 chromosomes because chromosome 21 appears three times instead of twice. This extra genetic material changes how the body and brain develop. The condition is not caused by anything a parent did before or during pregnancy.
How Do Babies Get Down Syndrome Causes Risk? The Reason
The extra chromosome 21 comes from the egg or the sperm that formed the baby. In about 90 percent of cases, the extra chromosome comes from the mother’s egg. In about 4 percent of cases, it comes from the father’s sperm. The remaining cases happen during early cell division after fertilization.
This error is called nondisjunction. It happens when chromosomes fail to separate properly during cell division. The egg or sperm ends up with two copies of chromosome 21 instead of one. When that cell joins with a normal cell from the other parent, the resulting embryo has three copies.
Nondisjunction is a random event. It is not inherited and it is not caused by anything the parents did. Researchers do not fully understand why it happens, but they know it becomes more likely as a woman’s age increases.
What Are the Three Types of Down Syndrome?
There are three genetic variations that cause Down syndrome. They all result in extra chromosome 21 material, but they happen in different ways.
Trisomy 21 is the most common type. About 95 percent of people with Down syndrome have this form. Every cell in the body has three full copies of chromosome 21 instead of two.
Translocation Down syndrome accounts for about 3 to 4 percent of cases. In this type, the extra chromosome 21 material is attached to another chromosome. The total chromosome count is 46, but there is still extra chromosome 21 material present. This form can be inherited from a parent who carries the rearranged chromosome without having Down syndrome themselves.
Mosaic Down syndrome is the rarest type, affecting about 1 to 2 percent of people. In this form, only some cells have the extra chromosome 21. Other cells have the typical 46 chromosomes. The physical features and developmental effects can vary widely depending on how many cells carry the extra chromosome.
Does Maternal Age Increase the Risk?
Maternal age is the strongest known risk factor for Down syndrome. The chance of having a baby with Down syndrome increases as a woman gets older. This is because a woman is born with all the eggs she will ever have, and those eggs age along with her. Older eggs are more likely to have errors during cell division.
The risk increases gradually and then more sharply after age 35. At age 25, the risk is about 1 in 1,200. At age 35, it is about 1 in 350. By age 40, it is about 1 in 100. At age 45, the risk rises to about 1 in 30.
These are population averages. They do not predict what will happen in any individual pregnancy. Most babies with Down syndrome are born to younger women simply because younger women have more babies overall.
Are There Other Risk Factors?
Paternal age may play a small role. Some studies suggest that fathers over 40 have a slightly increased chance of passing on the extra chromosome. The effect is much smaller than the maternal age effect, and the evidence is less consistent.
Having one child with Down syndrome increases the chance of having another. For most couples, the recurrence risk is about 1 percent. This risk is higher if one parent carries a chromosomal rearrangement, which is why genetic counseling is recommended for families who have had a child with Down syndrome.
No lifestyle factors are known to cause Down syndrome. The condition is not linked to diet, exercise, stress, medications, or environmental exposures during pregnancy. Parents should not blame themselves.
How Is Down Syndrome Diagnosed During Pregnancy?
Screening tests estimate the chance that a baby has Down syndrome. They do not give a definite answer. Diagnostic tests can confirm the diagnosis with near certainty.
Screening tests include blood tests and ultrasound measurements. The first-trimester combined screening measures two proteins in the mother’s blood and uses ultrasound to measure the fluid at the back of the baby’s neck. This test detects about 82 to 87 percent of Down syndrome pregnancies. Cell-free DNA testing, sometimes called NIPT, analyzes small pieces of the baby’s DNA in the mother’s blood. It detects about 99 percent of cases and has a very low false-positive rate.
Diagnostic tests include chorionic villus sampling and amniocentesis. These tests analyze the baby’s actual chromosomes. Chorionic villus sampling is done between 10 and 13 weeks of pregnancy. Amniocentesis is done between 15 and 20 weeks. Both tests carry a small risk of miscarriage, which is why they are not offered to every pregnant woman.
No screening or diagnostic test is mandatory. Whether to have testing is a personal decision that depends on your values, your medical history, and your preferences.
What Health Conditions Are Associated With Down Syndrome?
Children with Down syndrome are at higher risk for certain medical conditions. Knowing these risks helps families prepare and get early care.
Heart defects occur in about half of babies with Down syndrome. Some defects are minor and need no treatment. Others require surgery in the first year of life. Every baby with Down syndrome should have an echocardiogram shortly after birth.
Other common conditions include hearing loss, vision problems, thyroid disorders, and digestive issues. Children with Down syndrome also have higher rates of sleep apnea, ear infections, and celiac disease. Many of these conditions are treatable with routine medical care.
Life expectancy for people with Down syndrome has improved dramatically. In the 1980s, the average lifespan was about 25 years. Today, many people with Down syndrome live into their 60s and beyond. Most of this improvement comes from better treatment of heart defects and other medical conditions.
What Does Development Look Like for Children With Down Syndrome?
Every child with Down syndrome is an individual. Development varies widely from child to child. Some children learn to read and write. Others need more support with communication and daily living skills.
Most children with Down syndrome reach developmental milestones later than typical children. They may sit up later, walk later, and talk later. Early intervention services, including physical therapy, speech therapy, and occupational therapy, can make a meaningful difference in outcomes.
Intellectual disability ranges from mild to moderate for most people with Down syndrome. Many adults with Down syndrome live semi-independently, hold jobs, and form close relationships. The range of possibilities is wider than many people realize.
Frequently Asked Questions
Can Down syndrome be prevented?
No. There is no known way to prevent Down syndrome because the extra chromosome happens by chance during cell division. The only factor that clearly affects risk is maternal age.
Is Down syndrome inherited from the parents?
Most cases are not inherited. Trisomy 21 happens randomly during egg or sperm formation. Only the rare translocation form can be passed from a parent who carries a rearranged chromosome.
Do screening tests tell you for sure if your baby has Down syndrome?
No. Screening tests only estimate the chance. Only diagnostic tests like chorionic villus sampling or amniocentesis can confirm the diagnosis by examining the baby’s chromosomes.
Can a woman under 35 have a baby with Down syndrome?
Yes. Most babies with Down syndrome are born to women under 35 because that age group has the most pregnancies. The individual risk is lower, but the total number of cases is higher.
