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Genetic Counseling

 
 

Causes of Down Syndrome

Down syndrome is caused by congenital chromosomal abnormalities including nondisjunction, translocation, and mosaicism. The overwhelming majority of individuals with Down syndrome, approximately 95%, have trisomy 21, caused by a nondisjunction event during the formation of reproductive cells. This occurs when chromosome 21 fails to separate properly, resulting in sperm or egg cells with an extra chromosome 21. When such a gamete combines with a normal one, the embryo ends up with three copies of chromosome 21, leading to Down syndrome. A small number of individuals with Down syndrome, approximately 4% of all cases, have translocation Down syndrome. In these cases, the total number of chromosomes is the same as in a typical person, but a portion of chromosome 21 has broken off and incorrectly attached to another chromosome. About one-quarter of these cases occur because one parent is a balanced translocation carrier. While the parent's chromosomes contain no gain or loss of genetic material and do not cause Down syndrome, their offspring have a one-in-three chance of inheriting an unbalanced translocation. This results in an extra set of genetic material from chromosome 21, leading to Down syndrome. If a child is diagnosed with translocation Down syndrome, the parents are advised to undergo a karyotype analysis to determine whether they are balanced translocation carriers. For carriers, the recurrence risk varies depending on which parent is the carrier: 3% if the father is the carrier and 10–15% if the mother is the carrier. In extremely rare cases, approximately 1% of individuals with Down syndrome have mosaic Down syndrome. In this type, only some of the person’s cells have an abnormal number of chromosomes, while the rest are typical. As a result, individuals with mosaic Down syndrome often exhibit milder characteristics and clinical symptoms compared to other types. Their intelligence quotient (IQ) can reach up to 60, and the likelihood of congenital heart defects is significantly lower.

Prevention 

The likelihood of Down syndrome increases with maternal age. Women over 30 have approximately a 1 in 800 chance of giving birth to a baby with Down syndrome, while the risk rises to 1 in 25 for mothers over 45. While the occurrence is lower among younger mothers, every pregnancy carries some risk of having a baby with Down syndrome. Advances in medical testing have introduced prenatal screening and diagnostic procedures to assess the risk of Down syndrome and confirm its presence before birth. Prenatal testing for Down syndrome is categorized into two types: screening tests and diagnostic tests. 

Prenatal Screening

Prenatal screening is used to estimate the likelihood of the fetus having Down syndrome but cannot definitively confirm whether the fetus is affected. In contrast, prenatal diagnostic tests are used to determine the chromosomal profile of the fetus and provide a definitive diagnosis. Prenatal screening is generally non-invasive and combines maternal blood tests with fetal ultrasound results. These findings, along with the mother's age, are used to calculate the risk of the fetus having Down syndrome. A newer method, Non-Invasive Prenatal Testing (NIPT), analyzes free-floating fetal chromosomes in the mother's blood. This method offers a highly accurate risk assessment for Down syndrome.

Prenatal Diagnosis 

Prenatal diagnosis involves invasive procedures such as chorionic villus sampling (CVS), recommended between the 10th and 12th weeks of pregnancy, or amniocentesis, recommended between the 16th and 18th weeks. These methods obtain fetal cell samples, which are cultured and analyzed through chromosomal karyotyping. The accuracy of these tests is nearly 100%; however, they carry a miscarriage risk of 0.1% to 1%. Thanks to the implementation of maternal blood screening for Down syndrome, the prevalence of live births with Down syndrome in Taiwan has decreased to approximately 1.6 per 10,000 live births.