As the name suggests, Down’s syndrome screening is a prenatal screening test for Down’s syndrome fetuses, which is an important indicator for screening fetuses for trisomy 21 by taking blood tests from pregnant women. The results of the screening can determine whether the fetus has congenital mental defects and the risk of Down’s syndrome. If the screening result is high risk, further confirmatory amniocentesis will be required.
1. Basic overview
Down’s syndrome, also known as trisomy 21, is the most common chromosomal aneuploidy disorder in which the patient has one more than normal pair of chromosomes. Down’s syndrome screening is a risk value calculated by risk assessment software by measuring the levels of PAPPA, AFP, HCG, uE3 and InhibinA in the pregnant woman’s serum at a specific week of pregnancy combined with clinical information such as the pregnant woman’s age, gestational age, weight and whether she smokes and has insulin-dependent diabetes.
The threshold value is 1/250-380 which may vary depending on the methodology. A value greater than this is considered high risk and a value less than this is considered low risk. The probability of having Down’s DS in the general population under 35 years of age is 1/750. depending on the timing of the test, it is divided into 9-13 weeks of early pregnancy and 14-21 weeks of mid pregnancy.
Down screening is the abbreviation for prenatal screening for Down’s syndrome. The purpose of the test is to determine the risk of Down’s syndrome in the fetus by combining the blood of the pregnant woman with other clinical information. If the results of the screening test indicate a high risk of Down’s syndrome in the fetus, further confirmatory tests, such as amniocentesis or chorionic villus testing, should be performed.
2.Screening process
(1) First trimester
This test is performed during the 10th to 13th+6th weeks of pregnancy by ultrasound and blood sampling. Dr. Huang Xuguang said that the ultrasound can clearly measure the thickness of the nuchal translucency and the blood test can measure the plasma protein A and chorionic gonadotropin in the mother’s serum to estimate the risk of Down’s syndrome.
(2) Second trimester
The risk of Down’s syndrome is calculated by taking a blood sample at the 15th to 20th week of pregnancy to measure serum values of alpha-fetoprotein, chorionic gonadotropin, free estriol and inhibin, together with the mother’s age, number of weeks of pregnancy and weight.
(3) Chorionic villus sampling
Dr. Huang said that it is important to discuss this test with the doctor carefully because it is an invasive test that requires a sample of cells from the developing placenta. For example, if the nuchal translucency is more than 3 mm after the first trimester screening, direct chorionic villus sampling can be considered to test for chromosomes. However, the results of this test will not be known until 14 days later.
(4) Amniocentesis
This is also an invasive test but the risk is less than chorionic villus sampling. Dr. Huang Xuguang said that by taking the amniotic fluid from the mother’s uterus, the test can reveal whether there is any chromosomal abnormality in the fetus and thus the possibility of Down’s syndrome. It takes 14 days for the results to be known and the accuracy rate is over 99%.
3.History
Before 1984, amniocentesis was performed on pregnant women older than 35 years of age to detect chromosomal abnormalities in amniotic fluid.
In 1984, Dr. Merkatz, Dr. Cuckle & Wald found that the AFP in the blood of pregnant women with Down’s syndrome was about 15% lower than that of normal pregnant women.
In 1987, Dr. Bogart found that β-hCG in the blood of pregnant women with Down’s syndrome was higher than that of normal pregnant women.
In 1988, CanickHaddowWald found that uE3 in the blood of pregnant women with Down’s syndrome was decreased. The first triple screening protocol for children with Down’s syndrome was also proposed.
In 1996, inhibin A became the fourth serological test and proposed a quadruple screening program for Down’s syndrome, quadTest.
4.Related introduction
The best time for screening is at 1520 weeks of pregnancy.
It is best to consult your doctor before the test for other preparations.