Chromosomes are tiny structures found in every cell of our bodies. These structures hold the DNA genetic codes that dictate our natural traits and characteristics (e.g., hair and eye colour, height).
There are 23 pairs of chromosomes in each human cell (i.e., 46 individual chromosomes), including one pair of sex chromosomes. Down syndrome is a genetic condition caused by the presence of an extra whole chromosome, or part of a chromosome, specifically chromosome 21.
Although Down syndrome is a genetic condition, it is not inherited. In Canada, approximately 1 in 750 babies are born with Down syndrome. This risk increases with increasing maternal age.
According to the NDSS and the National Association for Down Syndrome (NADS), there is no connection between any type of Down syndrome and parents' activities before or during pregnancy. It is important to know that nothing the mother does during pregnancy could cause Down syndrome. However, we do know that the chance of having a baby with Down syndrome increases significantly with maternal age, and with teenage pregnancies. Since many couples are postponing pregnancy until later in life, the incidence of Down syndrome conceptions may increase. Therefore, genetic counselling for parents may become more important.
The most common type of Down syndrome is trisomy 21, which accounts for 95% of all cases. In this type of Down syndrome, abnormal cell division results in an extra chromosome 21 in each cell. Prior to or at conception, a pair of chromosomes 21 (in either the sperm or the egg) fails to separate. As the embryo develops, the extra chromosome is replicated in every cell of the body. At present, researchers are not sure what causes the presence of an extra chromosome 21. They do know that it can come from either the mother or the father. In a person with Down syndrome, each cell makes excess amounts of chemicals regulated by the chromosome 21 genes. This imbalance in part explains physical effects and biological changes noted in people with Down syndrome.
Other less common forms of Down syndrome result from other processes occurring during cell division, which result in an extra partial chromosome. 2% to 3% of people with Down syndrome have a type called translocation, and approximately 2% of people with Down syndrome have a type called mosaicism.
Symptoms and Complications
There are several physical characteristics that are more common for babies with Down syndrome. These may include a flat facial profile, almond shaped eyes with an upward slant, unusual eyelids (known as epicanthic folds), a flat nasal bridge, a prominent tongue, small ears, single crease on the palms of the hands (known as a palmar crease), smaller limbs, smaller body frame, and a small head. Even though they may have these characteristics, babies and children with Down syndrome may still look much like other family members.
Health and development
Children with Down syndrome may have other health complications besides the usual childhood illnesses. No individual with Down syndrome will have all the health complications. However, every child needs supportive, preventative medical care. Early and proper health care can help people with Down syndrome better manage their condition.
Some health problems are less serious while others can be very problematic, such as malformations of the heart. According to the Canadian Down Syndrome Society (CDSS), more than 40% of children born with Down syndrome have a congenital heart malformation that's present at birth. However the diagnosis of a heart problem does not mean the baby is or will become seriously ill, and the extent of the heart problem varies from very mild to life-threatening. In addition, about 10% of newborns with Down syndrome will have a blockage in the intestines that requires surgery. Some degree of hearing loss (most often due to frequent ear infections) can affect children with Down syndrome.
Babies with Down syndrome can be breast-fed, and breast-feeding mothers may find it helpful to talk with their doctor or nurse to prepare for the process. Babies and children with Down syndrome also have an increased risk of developing hypothyroidism (low thyroid hormone), diabetes, leukemia, and seizure disorders. However, early detection and treatment can help manage these conditions.
Most children with Down syndrome will have speech delay, which can be further aggravated by hearing loss. In these cases, a speech therapist can play an important role in the child's learning and development. Visual problems (e.g., cataracts, crossed eyes, or far- or nearsightedness) are also more common for children with Down syndrome and occur in around 60% of cases. In some cases, corrective eyewear and medications may help to improve vision, while other children may require surgery to resolve visual problems. Due to poor muscle tone, babies with Down syndrome may have trouble rolling over, sitting up, and walking, and they may require regular physiotherapy.
For adults with Down syndrome, infertility is common for men. Pregnancies are possible for women with Down syndrome, although there is an increased risk of having a child with Down syndrome. Genetic counselling and family planning specialists are usually involved to educate and provide support where needed. Adults with Down syndrome also face an increased chance of developing early-onset Alzheimer's disease. They are also more likely to be obese in adulthood.
Learning and development
Levels of learning ability for people with Down syndrome can range from near-normal to severe disability. While most people with Down syndrome have mild-to-moderate learning impairment and developmental delays, the degree of impairment is not predictable either before or after birth. The extra chromosome means that your baby or child will likely be slower to reach developmental milestones and will have a certain degree of learning difficulty.
Like other children, most children with Down syndrome learn to walk and talk, ride a bicycle, read, and have a wide range of abilities and talents. If your child has Down syndrome, a base of good physical health and continued social support can help foster your child's ability to learn.
Effects on the family
Having a baby with Down syndrome may require a period of adjustment for the family. Parents may feel more pressure to focus on the development of their child with Down syndrome over their other children. Take time to get to know your child as part of the family, and keep in mind that he or she will have a unique personality and will require plenty of love and support from the parents, just like other children.
Making the Diagnosis
The diagnosis of Down syndrome is usually suspected after birth, as a result of the baby's appearance. However, to confirm a diagnosis, a test called a karyotype (chromosome study) is performed. This test involves taking a sample of blood from the baby to look at the chromosomes within the cells.
It may be a difficult time of waiting for test results coupled with the natural stresses of childbirth and parenting. Parents of children newly diagnosed with Down syndrome may feel a range of emotions, such as sadness, shock, and guilt. Although learning of the diagnosis may be difficult at first, some families agree that being together during a diagnosis and being informed as soon as possible makes it easier to begin caring for their child.
There are tests that doctors can perform before a baby's birth to screen for or diagnose Down syndrome. These tests are called prenatal tests. Screening tests estimate the risk of the fetus having the Down syndrome, while diagnostic tests indicate whether or not the fetus actually has Down syndrome. These screening tests are usually offered between 10 and 20 weeks into the pregnancy.
The most common screening tests measure quantities of various substances in the blood and, coupled with the woman's age, estimate a woman's risk of having a child with Down syndrome.
Other tests use a combination of ultrasound markers, such as thickness of the baby's skin at the back of the neck, together with blood tests in the first or second trimester (or both) of pregnancy, as well as the mother's age. These tests have the advantage of detecting more cases, while having a lower false-positive rate (i.e., a positive test result although the pregnancy later proves to be normal). When the test result is positive, further diagnostic tests (see below) are necessary to be certain whether or not the fetus has Down syndrome. Pregnancies affected with Down syndrome will screen positive close to 90% of the time.
For diagnostic tests, there are two methods currently available: amniocentesis and chorionic villus sampling (CVS). These procedures are about 98% to 99% accurate in detecting Down syndrome, though each carries a small risk of miscarriage. This is because samples are taken from the amniotic fluid or the developing placenta. An amniocentesis is usually performed between Week 15 and Week 22 of the pregnancy. CVS is performed between Week 9 and Week 14. The results of these tests are available within 2 to 3 weeks.
Before undergoing any of these diagnostic tests, women and their partner should seek genetic counselling to discuss the risks and benefits of these procedures.
Treatment and Prevention
Many adults with Down syndrome have jobs and live semi-independently, and people with Down syndrome are becoming increasingly integrated into society and community organizations. A greater understanding of Down syndrome and advancements in the treatment of related health problems have allowed people with Down syndrome to enjoy full and more active lives. Children with Down syndrome who are raised at home and included in all aspects of community life can best reach their potential and achieve greater levels of independence.
Early and adequate health care can protect people with Down syndrome from most of the risks associated with the syndrome, such as heart problems or a thyroid disorder. With recent advancements in treatment, most adults with Down syndrome reach the age of 55, and some may live even longer.
As mentioned earlier, testing for Down syndrome during pregnancy is possible, and it allows parents to consider their options in advance. Parents of children with Down syndrome usually find that sharing their feelings brings reassurance, comfort, and support. Contact your local Down syndrome association or organization for information about support groups or social counsellors.