Down syndrome is a genetic condition resulting from an extra chromosome. It occurs due to a random error in cell division, leading to additional genetic material. Understanding its genetic basis and influencing factors clarifies why traditional prevention methods are not applicable. While not preventable, prenatal tests allow for identification during pregnancy, providing families with information to prepare.
What is Down Syndrome
Down syndrome is a genetic condition characterized by an extra copy of chromosome 21. Typically, human cells contain 23 pairs of chromosomes, totaling 46. In individuals with Down syndrome, there are three copies of chromosome 21 instead of two, a condition known as Trisomy 21. This additional genetic material alters development, influencing how the brain and body form.
The most common cause of Down syndrome is a random error in cell division called nondisjunction, accounting for about 95% of cases. Nondisjunction occurs when a pair of chromosome 21 fails to separate properly during the formation of reproductive cells (sperm or egg). An egg or sperm cell may end up with an extra chromosome 21, and when this cell combines with a normal reproductive cell, the resulting embryo has three copies of chromosome 21.
Less common types include Translocation Down syndrome, where an extra part or whole chromosome 21 attaches to another chromosome, accounting for about 3% of cases. Mosaic Down syndrome is the rarest form, affecting less than 1% of individuals, and occurs when only some cells in the body have an extra chromosome 21, leading to a mixture of cells with 46 and 47 chromosomes. These genetic events are generally random and not caused by external factors.
Factors That Influence Occurrence
Certain factors can influence the likelihood of Down syndrome. Advanced maternal age is the most significant factor associated with an increased chance of having a child with Trisomy 21. The risk increases gradually until about age 30, and then more substantially thereafter, as older eggs have a higher chance of unusual chromosome division during meiosis. For example, the risk for a 25-year-old woman is approximately 1 in 1,250, increasing to about 1 in 350 at age 35, and around 1 in 100 by age 40.
Despite this, most babies with Down syndrome are born to mothers under 35 due to higher birth rates in younger age groups. It is important to note that these are statistical correlations, indicating a higher probability. Down syndrome can occur in any pregnancy, regardless of parental age or family history, as most cases arise from spontaneous genetic changes.
Identifying Down Syndrome During Pregnancy
Various tests are available for identifying Down syndrome during pregnancy. These tests provide information to families and are broadly categorized into screening and diagnostic tests.
Screening tests are non-invasive and estimate the probability of Down syndrome. These include blood tests measuring specific hormone levels, such as beta-human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein-A (PAPP-A), typically performed during the first trimester. An ultrasound measurement of nuchal translucency, the thickness of fluid at the back of the baby’s neck, is often combined with these blood tests. Non-invasive prenatal testing (NIPT) is another screening option, analyzing fragments of fetal DNA in the mother’s bloodstream as early as 10 weeks of pregnancy. NIPT is highly sensitive, identifying over 99% of Down syndrome cases, but does not provide a definitive diagnosis.
For a definitive diagnosis, invasive diagnostic tests are necessary. Chorionic villus sampling (CVS) can be performed between 10 and 13 weeks of pregnancy, collecting a tissue sample from the placenta. Amniocentesis, usually done after 15 weeks, involves taking a sample of the amniotic fluid. Both CVS and amniocentesis carry a small risk of complications, such as miscarriage, but offer highly accurate results by directly analyzing the baby’s chromosomes.
Is Prevention Possible
Down syndrome is a chromosomal condition that arises from a random error in cell division. It is not the result of anything a parent did or did not do before or during pregnancy.
Currently, no known preventative measures exist for Down syndrome. The focus remains on early identification, which allows prospective parents to gain information about the condition. This early understanding can help families prepare for the birth of a child with Down syndrome and access support services.