Risk Values for Trisomy 21

Prenatal Diagnosis: What Is It For?

Prenatal Diagnosis—First-Trimester Combined Screening—allows for the determination of the probability of trisomy 21 (Down syndrome) in the mother’s blood, as well as the presence of other genetic abnormalities such as trisomy 18 (Edwards syndrome) or trisomy 13 (Patau syndrome) in the developing fetus.

This test also helps parents make informed decisions, while enabling healthcare professionals to provide appropriate care and intervention strategies starting at birth.

Continue reading this article to learn about the risk values for trisomy 21, a genetic condition resulting from the presence of an extra chromosome in chromosome pair 21.

Edwards Syndrome and Patau Syndrome: What Are They?

Edwards syndrome is a genetic disorder characterized by intellectual disability, growth retardation, and, at times, severe heart malformations. The skull is excessively elongated in the occipital region, the outer ears have few folds, the mouth is small, and the neck is usually very short.

There is a wide distance between the nipples, and the external genitalia are abnormal. The index finger is larger than the others and bent over the middle finger.

Patau syndrome includes severe malformations of the central nervous system, marked mental retardation, congenital heart defects, and urogenital defects. Cleft lip and cleft palate, clenched fists, a sloping forehead, ocular hypertelorism, and bilateral microphthalmia are frequently observed. Anophthalmia, iris coloboma, and small eyes that are extremely widely spaced or absent may also occur.

The ears are malformed and low-set. The hands and feet may have a sixth finger or a fifth finger overlapping the third and fourth fingers.

Prevalence of the Syndromes

The prevalence of these syndromes varies according to maternal age. Consequently, the risk of trisomy 21 may increase as a woman gets older.

While the risk of Down syndrome affects approximately 1 in every 700 births, Edwards syndrome occurs in 1 in every 8,000 births, and Patau syndrome occurs in 1 in every 20,000 newborns.

Learn about the signs of trisomy 21 during pregnancy.

Assessment of the risk of trisomy 21

For all these reasons, it is important to undergo the First-Trimester Combined Prenatal Screening during pregnancy.

Maternal age and ultrasound have a detection rate of 75%. And if a biochemical test is also performed, the first-trimester combined screening can detect between 90% and 97% of cases.

Thus, combining these tests can provide a more comprehensive and accurate assessment of the risk of trisomy 21 during pregnancy.

First-Trimester Combined Screening: What Does It Assess?

The purpose of the First-Trimester Combined Prenatal Screening test is to assess the risk of trisomy 21 in the fetus. However, it does not completely rule out the possibility of trisomy 18 or trisomy 13, nor does it entirely eliminate the chance of birth defects or other problems that cannot be detected through prenatal screening tests.

This test should be performed between the 10th and 13th week plus 6 days of gestation, calculated from the date of the last menstrual period. During the test, various biochemical markers will be measured, and maternal blood will be analyzed to determine the risk of trisomy 21.

If the result indicates a high risk, additional tests may be recommended to confirm the diagnosis. It is also important to note that the test result is not a definitive diagnosis, but rather an assessment of probability.

Definitive Diagnosis and Associated Risks

For a definitive diagnosis—and if a risk of trisomy 21 is identified—it is necessary to use invasive diagnostic methods, such as chorionic villus sampling (CVS) or amniocentesis. These procedures allow for the collection of fetal cells for a more accurate analysis.

However, it is important to be aware that invasive methods also carry a small risk of miscarriage, ranging from 0.5 to 1%. Therefore, it is essential to discuss all available options with your doctor and the risks involved before making a decision.

Decision-Making Support

Given the risk values for trisomy 21 presented above, we emphasize the importance of first undergoing the First-Trimester Combined Prenatal Screening to make an informed decision before proceeding with invasive diagnostic methods.

Down syndrome has distinct characteristics and unique impacts on the lives of affected individuals. Understanding and recognizing the specific characteristics of each syndrome—in order to provide appropriate support and care for children with trisomy 21—is essential for improving their quality of life and social integration.

In this regard, the Germano de Sousa Laboratory Medicine Center offers the best service, as it is certified by the Fetal Medicine Foundation (FMF) to perform this test and serves as a reference laboratory in this field.

Do you have more questions about prenatal screening? Visit our page “Questions and Concerns About Prenatal Screening” to find answers to all your questions.