Why is Down syndrome screening done twice?

In order to reduce the occurrence of various deformed babies, various pregnancy examinations can now be used to check for various deformed fetuses. Down syndrome testing can determine whether the fetus has the possibility of Down syndrome, so this test is an important part of prenatal examinations. So does Down syndrome screening need to be done twice?

Does Down syndrome screening need to be done twice?

Whether Down syndrome screening needs to be done twice depends on the detailed situation of the pregnant woman. There is no requirement for the actual frequency. Down screening is the general name for prenatal selection tests for Down syndrome. The purpose is to test the blood of pregnant women, test the concentrations of alpha-fetoprotein, human gonadotropin and free estriol in the blood cells of pregnant women, and combine the age, weight, and weeks of pregnancy of the pregnant women to determine the risk of congenital idiocy and neural tube defects in the fetus.

Down syndrome screening is a test method that extracts blood cells from pregnant women, tests the concentration of alpha-fetoprotein and human gonadotropin in the mother's blood cells, and calculates the risk of having a child with Down syndrome based on the pregnant woman's due date, age, weight and weeks of gestation when the blood is drawn.

The best time to do Down syndrome screening is between 15 and 20 weeks of pregnancy. Doctors recommend that Down syndrome screening should be done at 15 to 20 weeks, but each testing hospital has different conditions, so the actual time for Down syndrome screening should be announced by the hospital. This step can predict the possibility of congenital idiocy through examination. The main purpose of Down syndrome screening is to avoid the risk of congenital idiocy in the fetus to a certain extent. Generally, pregnant women can get the screening results within one week after the blood test. If the result is high risk, there is no need to panic, because further amniocentesis and fetal chromosome examination are required to confirm the diagnosis.

Who needs Down syndrome screening?

1. The factor of mother’s age

The probability of the fetus developing this disease is directly proportional to the age of the mother. According to reference literature reports, the incidence of this disease in pregnant women >30 years old is 1/1000. For those aged >35, it is 1/300. For those aged 35-39, it is 1/50. For those aged >40, the rate is 1/20. Therefore, women should avoid getting pregnant after the age of 40, and women over the age of 35 should pay attention to contraceptive surgery. Genetic counseling and prenatal screening should also be enhanced for young mothers under 30 years old.

2. Necessity of sex chromosome diagnosis before pregnancy

Examining chromosomes is the key way to diagnose this disease. According to cell biology standards, there are three types of congenital idiocy: (1) basic type: 47, XX (or XY) 21, accounting for 95%. (2) Chromosome translocation type: D/G chromosome translocation and G/G chromosome translocation, accounting for 2.5% to 5%. (3) Chimera type, accounting for 2% to 4%. Among them, there are two most common cell lines in the body. If the mother has a patient or has such a disease in the allele, the chromosomes should be checked and the probability of developing the disease should be predicted based on the cytobiological types to guide whether she can become pregnant again or not. If you are pregnant, you should immediately undergo a pregnancy check and terminate the pregnancy if necessary.