One of the biggest risks associated with advanced maternal age of 35 or older is the risk that your child may be born with Down syndrome. Each year, 1 in 700 babies are born with Down syndrome, ranking it as the most common chromosomal disorder.
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A healthy cell will contain 23 pairs of chromosomes or 46 chromosomes in total. An individual with Down syndrome will have cells that contain 47 chromosomes. This extra chromosome causes a change in the way the brain and body are developed. The extra chromosome can appear in 3 ways:
- a) Nondisjunction or trisomy 21 – Nondisjunction, commonly referred to as trisomy 21, is the most common chromosomal anomaly seen in children with Down syndrome, accounting for 95% of cases. When a nondisjunction occurs, 3 copies of the chromosome 21 are passed on to the baby instead of 2. The third copy of chromosome 21 appears when a pair of chromosome 21’s, from either the sperm or the egg, do not separate during meiosis, resulting in every cell carrying the extra chromosome 21.
- b) Mosaicism – Mosaicism accounts for about 1% of Down syndrome cases. In this case, there are cells that contain the healthy 46 chromosomes and cells that also contain 47 chromosomes with the extra chromosome 21. Mosaic trisomy 21 occurs after the junction of the egg and sperm during early development of the fetus. This allows for a mixture of healthy cells and abnormal cells.
- c) Translocation – Translocation accounts for about 4 % of Down syndrome cases. This occurs when part of a chromosome 21 breaks off during cell division and attaches itself to another chromosome. The symptoms and severity of individuals with a translocation and a nondisjunction are indistinguishable.
Diagnosing Down Syndrome Prenatally
At your first appointment with your health care physician, you may be offered fetal trisomy testing or other prenatal genetic tests, which will allow you and your physician the ability to understand if your child is at risk of developing Down syndrome, or other trisomies or genetic conditions. Your physician will analyze the results of the screening as well as your family history to determine the risk.
If the screening shows that your baby is at a high riskof developing Down syndrome, your physician will then suggest a diagnostic test. Prenatal genetic testing can provide more information on the likelihood of your child having Down syndrome.
If you choose to forego testing during your pregnancy, your physician may diagnose Down syndrome after the birth of your child by observing these traits:
- Eyes that slant upwards
- Small skin folds may be present on the inner corner of the eyes
- Small nose and flat nasal bridge
- Low muscle tone
- Too much space between the large and second toes
Each characteristic may manifest itself differently in each individual diagnosed with Down syndrome. Not every individual with Down syndrome will exhibit all of these traits, and the degree to which each trait is expressed will vary.
For couples who suspect that they may have higher likelihood of giving birth to a baby with Down syndrome, it is a good idea to speak with your physician and genetic counselor so you can better understand the condition and decide if prenatal tests should be considered.