Down Syndrome Screening Test

Down syndrome (Trisomy 21) is a genetic disorder caused by the presence of an extra part or all of the 21st chromosome. Children with Down syndrome may experience mild to severe intellectual disability, learning difficulties, and delays in physical development. Common physical characteristics include slanted eyes, shorter arms and legs, and speech problems. They are also at a higher risk for heart anomalies, reflux, recurrent ear infections, sleep disorders, and thyroid diseases. As a genetic disorder, Down syndrome can be detected through a screening test performed during pregnancy. On average, Down syndrome occurs in 1 in 800 to 1,000 births.

Screening tests are assessments conducted on the entire population to identify certain diseases in healthy individuals. These tests enable early intervention before a disease manifests or in its early stages. Examples of screening tests in women's health include mammograms for breast cancer and PAP smears for cervical cancer. However, a screening test is not a diagnostic test. If a screening test indicates a positive or high-risk result, further tests are required to confirm the diagnosis. For example, a suspicious area on a mammogram may require a tissue sample (biopsy), or a biopsy may be needed following a smear test from the cervix.

Screening tests are low-cost and cause minimal discomfort to individuals, allowing them to be applied to all individuals. In contrast, diagnostic tests require special procedures, expertise, and laboratory evaluations, making them more expensive. For example, to diagnose Down syndrome, amniocentesis (the procedure to collect a sample of amniotic fluid) must be performed. This requires an experienced specialist, ultrasound, and genetic laboratory analysis. There is also a risk of miscarriage associated with the procedure. Due to the risks and costs, only pregnancies reported as "high risk" by screening tests are recommended for diagnostic testing.

Chromosomes are the building blocks that carry the information that makes us who we are and passes this information to our children. Humans have 23 pairs of chromosomes, one of which consists of sex chromosomes (XY for males; XX for females).

Between the 10th and 14th weeks, as well as between the 16th and 18th weeks of pregnancy, the mother's blood is taken to conduct biochemical screening. During the 10th to 14th weeks, levels of free beta hCG and PAPP-A are measured, while levels of alpha-fetoprotein, beta hCG, and estriol are measured during the 16th to 18th weeks. These tests are not "intelligence tests." As you know, intelligence tests are evaluated through written and verbal assessments, not through blood values. The term "intelligence test" is often used incorrectly.

Obstetricians define the pregnancy period in three trimesters. The first 14 weeks are called the first trimester. During the 10th to 14th weeks of pregnancy, your doctor will use ultrasound to measure the baby's age, heartbeat, and nasal bone. During this time, the thickness of the nuchal translucency (NT) at the back of the baby's neck is also measured. The baby's age, NT measurement, and blood tests (free beta hCG and PAPP-A) are used to estimate risk.

During the second trimester (the second three-month period), blood is also taken from the mother between the 16th and 18th weeks to conduct tests (alpha-fetoprotein, beta hCG, and estriol). Risk calculations will also be provided based on the results of this test.

The primary tests used to diagnose Down syndrome include invasive methods such as chorionic villus sampling (CVS), amniocentesis, and cordocentesis. CVS is performed by taking a sample from the placenta (the baby's support structure) between the 8th and 12th weeks of pregnancy. Cordocentesis, which involves taking a blood sample from the baby's umbilical cord, is typically preferred during later stages of pregnancy (around 18-20 weeks) due to its rapid results.

Amniocentesis is performed between the 14th and 18th weeks of pregnancy by taking a sample of amniotic fluid. In addition to these invasive tests, recent methods involve taking a blood sample from the mother to detect fetal cells or chromosomal structures that have passed into the mother's blood, assisting in the diagnosis of Down syndrome.

The primary goal in diagnosing Down syndrome is to clarify the diagnosis before the baby has a chance to survive outside the womb, providing the family with sufficient time to make a decision regarding pregnancy termination. With today’s technology, the survival chance for a baby is considered to be around the 24th week of pregnancy. However, this developing method has limited diagnostic power.

Having discussed the nature of screening tests, what do high risk and low risk mean? Screening tests are not diagnostic; their purpose is to apply diagnostic tests (chromosomal analysis) and interventions (amniocentesis) to the fewest number of high-risk individuals possible.

A report indicating “high risk” or “positive” for Down syndrome does not mean that your baby has Down syndrome. Similarly, a “low risk” or “negative” report does not guarantee that your baby will not have Down syndrome. At best, with this screening logic and methods, nine out of ten babies with Down syndrome will be identified, but one will still be born. This statistic is consistent across all centers worldwide that apply similar screening policies.

Amniocentesis is not a screening test but a diagnostic test. It is perhaps not commonly mentioned that it is neither financially nor health-wise “economical” for the pregnant individual. Of every 1,000 procedures performed, 2-5 cases may result in pregnancy loss. Therefore, amniocentesis is only recommended for pregnancies reported as “high risk” by screening tests. Additionally, amniocentesis may be performed on pregnant individuals who understand and accept the risks involved and have discussed all outcomes with their doctor.

In multiple pregnancies, the values in the mother’s blood will vary based on the number of pregnancies. Therefore, the mother's blood values are not used for screening purposes in multiple pregnancies. While the NT and nasal bone measurements can be evaluated between the 10th and 14th weeks of pregnancy, and ultrasound findings related to Down syndrome can be assessed in the second trimester, it is important to remember that ultrasound evaluations alone have less diagnostic adequacy than tests performed on the mother's blood.

With first-trimester NT measurements, it is estimated that seven out of ten Down syndrome cases can be identified in multiple pregnancies. In multiple pregnancies, diagnostic procedures are performed on all fetuses if a high risk is identified after NT measurement.

It is important to have your screening tests done during the specified weeks of pregnancy and discuss the results with your doctor. Wishing you a healthy pregnancy.