Frequently Asked Questions
Down syndrome is caused by extra genetic material from chromosome 21. Chromosomes are the structures in cells that contain the genes. There are three types of Down syndrome: Trisomy 21, Translocation, and Mosaicism.
Each person normally has 23 pairs of chromosomes or 46 in all. An individual inherits one chromosome per pair from the mother's egg and one from the father's sperm. When an egg and sperm cell join together, they normally form a fertilized egg with 46 chromosomes.
Down syndrome is one of the most common genetic birth defects, affecting about 1 in 800 babies (1). According to the National Down Syndrome Society, there are approximately 350,000 individuals with Down syndrome in the United States (2).
The three types of Down syndromes are as follows:
Trisomy 21: A developing egg or sperm cell may divide incorrectly, sometimes resulting in an egg or sperm cell with an extra chromosome number 21. When this cell joins with a normal egg or sperm cell, the resulting embryo has 47 chromosomes instead of 46. Down syndrome also is called trisomy 21 because affected individuals have three number 21 chromosomes, instead of two. This type of error in cell division causes about 95 percent of the cases of Down syndrome
Translocation: Occasionally, before fertilization, a part of chromosome 21 breaks off during cell division and becomes attached to another chromosome in the egg or sperm cell. The resulting embryo may have what is called translocation Down syndrome. Affected individuals have two normal copies of chromosome 21 plus extra chromosome 21 material attached to another chromosome. This type of error in cell division causes about 3 to 4 percent of the cases of Down syndrome. In some cases, the parent has a rearrangement of chromosome 21, called a balanced translocation, which does not affect his or her health.
Mosaicism:About 1 to 2 percent of individuals with Down syndrome have a form called mosaicism. In this form, the error in cell division occurs after fertilization. Affected individuals have some cells with an extra chromosome 21 and others with the normal number.
The vast majority of cases of Down syndrome are not inherited. Only in cases of Translocation Down syndrome and then in only 1 of 3 cases of this type of Down syndrome is the condition inherited. These inherited cases occur because one of the parents is a carrier. It is important that parents have genetic counseling to determine their risk.
Many babies with Down syndrome have certain physical characteristics which are common in Down syndrome. These signs occur in up to 80% of all cases. Individual characteristics are often found in children without Down syndrome but the child with Down syndrome will have several. Please remember, your child with Down syndrome may have only some of these traits. These traits are:
the eyes have an upward and outward slant
there is a fold of skin on the inner side of the eye (epicanthal fold)
the eye slit is narrow and short
small, white patches can be seen on the edge of the iris
the face has a flat appearance
the head is smaller than average
the soft spots on the head (fontanels) are larger than normal because the baby is growing more slowly
the ears are smaller and lower-set
the mouth is small and the lips are thin which leads to the tongue sticking out because the inside of the mouth is smaller
the neck appears slightly short and loose folds of skin are seen at the back and sides (these go away as the baby grows)
the legs and arms are short in relation to the body
the hands are broad and flat with short fingers, the little finger slants inward, and there is a single crease across the palm
the feet are broad with short toes and there is a larger space than normal between the big toe and the other toes
there is poor muscle tone (hypotonia) and loose-jointedness (hyperflexibility)
reflexes tend to be weaker and the cry is weak
If a child has six to ten of these signs, the doctor can be almost certain that the child has Down syndrome, and a blood test to analyze the chromosomes will tell for sure.
Simple answer: NO! There is nothing you can do to prevent Down syndrome and nothing you can do to change your odds of having a child with Down syndrome. Down syndrome occurs in all races, in all socio-economic conditions, and in all countries. No relationship between diet or illness and Down syndrome has ever been found. The only established relationship is with maternal age. A woman is more likely to have a child with Down syndrome as she grows older with a sharp rise in risk at 35-40 years of age. However, 80% of all children with Down syndrome are born to mothers under 35.
In general, in each subsequent pregnancy, the chance of having another baby with Down syndrome is 1 percent plus whatever additional risk a mother has, based upon her age. If, however, the first child has translocation Down syndrome, the chance of having another child with Down syndrome may be greatly increased.
There is no cure for Down syndrome, nor is there any way to prevent it. Does the risk of Down syndrome increase with the mother's age?
Yes. The risk of Down syndrome increases from about 1 in 1,250 at age 25, to 1 in 1,000 at age 30, 1 in 400 at age 35, 1 in 100 at age 40 and 1 in 30 at age 45 Women over age 35 have been traditionally considered most likely to have a baby with Down syndrome. However, about 80 percent of babies with Down syndrome are born to women who are under age 35, as younger women have far more babies (2).
With rare exceptions, men with Down syndrome cannot father a child (3). In any pregnancy, a woman with Down syndrome has a 50-50 chance of conceiving a child with Down syndrome, but many fetuses are miscarried.
Most of the health problems associated with Down syndrome are treatable. Individuals with Down syndrome are more likely than unaffected individuals to have one or more of the following health conditions:
Thyroid problems, leukemia and seizures
Some individuals with Down syndrome may have a number of these problems, while others may have none. The severity of these conditions varies greatly.