Down Syndrome

Down syndrome, or trisomy 21, is the most common chromosomal disorder, with an incidence of about 1 in 700 live births. Since definitive diagnostic tests for Down syndrome are invasive and costly, it is not feasible to conduct them for all pregnant women. For this reason, screening tests to identify mothers at high risk were developed years ago. Through these tests, mothers at higher risk are identified and referred for definitive diagnostic testing. Since more than two-thirds of Down syndrome cases occur in mothers under 35 years of age, the American College of Obstetricians and Gynecologists (ACOG) strongly recommends that all pregnant women undergo Down syndrome screening.

Down Syndrome Screening Tests

First-Trimester Screening Test

This test is conducted between weeks 11 and 14 of pregnancy. It measures two blood markers and one ultrasound marker, and with computerized analysis, calculates the risk of Down syndrome, as well as trisomies 13 and 18 at birth. If this risk exceeds a specific threshold, the individual is referred for amniocentesis. The detection rate of this test is around 90%.

Note: Since blood marker levels are related to gestational age, it is essential to conduct an ultrasound to accurately determine the fetus’s age before the test.

Second-Trimester Screening Test

Today, the most common Down syndrome screening test performed worldwide on all pregnant women is the triple marker or quadruple marker test, ideally conducted between weeks 16 and 18 of pregnancy. In this test, three or four blood markers are measured. Using the obtained values and computerized analysis, the likelihood of Down syndrome, trisomies 18 and 13, and neural tube defects in the baby is calculated. The threshold used to distinguish between normal and high-risk cases in the quadruple marker test is a risk ratio of 1 in 270. This means that if the Down syndrome risk is reported as 1 in 270 or higher, the individual is referred for amniocentesis. The quadruple marker test has a detection rate of about 80% to 85%, which is higher than the triple marker test, and has a false-positive rate of 5%.

Amniocentesis

Amniocentesis involves obtaining amniotic fluid (the fluid surrounding the fetus), and it is used as a definitive diagnostic test if screening tests indicate high risk. The fetal cells in this fluid are examined chromosomally, and if the fetus has a chromosomal abnormality, the specific type is identified.

If a chromosomal abnormality is confirmed in the fetus, the pregnancy is terminated. In Iran, a pregnancy termination is permitted only if the gestational age is under four months.