Down’s syndrome screening is a method of assessing the risk of Down’s syndrome (trisomy 21) in the fetus of a pregnant woman using an economical and simple method that is non-invasive to the fetus. Screening methods include both serological and ultrasonographic screening, as well as a combination of both. At present, the more mature screening conducted in the country and even in our city is the serological screening in the middle of pregnancy.
1. Screening time: 15 to 20 weeks plus 5 days of pregnancy.
2. Screening method: 3mL of venous blood is taken from the pregnant woman (fasting is not required) and the blood is tested for alpha-fetoprotein and chorionic gonadotropin. The results of the test, as well as the age and gestational age of the pregnant woman, are entered into a computer and analyzed by specialized software used internationally to obtain the risk assessment value for the development of trisomy 21.
3. Types of diseases screened: In addition to trisomy 21, screening for trisomy 21 can also screen for trisomy 18 and neural tube defects (mainly anencephaly and open spina bifida), i.e. NTD.
The cut-off value for the risk of trisomy 21 is R1:270, and anyone with R1:270 is at high risk for screening. The cut-off value for the risk of trisomy 18 is R1:350, where R1:350 is the high risk of screening and <1:350 is the low risk. Any R1:350 is considered low risk and <1:350 is considered low risk.
The screening index for neural tube defects is the MOM value of AFP, which is R2.5 for high risk and 2.5 for low risk.
5. Factors affecting screening results.
(1) The gestational week must be accurate and determined by ultrasound if necessary. The accuracy of the gestational week is the most important factor that affects the accuracy of the screening results. (2) The age of the pregnant woman, such as some pregnant women’s ID card does not match the actual year of birth, will also affect the screening results.
(3) maternal weight, due to the dilution effect of the screening index level, but the general size of pregnant women, weight does not have a significant impact.
(4) Pregnant women who smoke will increase AFP by 3% and decrease hCG by 3%, which will affect the screening results.
(5) Racial differences: AFP levels are higher in Asians and blacks than in Hispanics and whites.
6. Explanations for the high risk of screening for trisomy 21.
(1) Trisomy 21 can occur in fetuses with perfectly normal parents and no family history. Because only the occurrence of translocation type 21-trisomy is genetically related, accounting for 5% of children with trisomy 21, the remaining 95% of children with trisomy 21 occur independently of genetics.
(2) High risk of trisomy 21 and trisomy 18 only indicates that the fetus is more likely to have these two congenital anomalies, but not a definitive diagnosis, and its amniocentesis is recommended for amniotic fluid fetal karyotyping to rule out chromosomal disorders. screening for trisomy 21 between 1:275 and 1:800 and trisomy 18 between 1:355 and 1:800, although it does not require Although amniocentesis is not required for fetal chromosome testing, a systematic ultrasound examination, or “4D ultrasound” as it is known, should be performed. If the screening risk is <
1:800, only regular prenatal checkups are needed.
(3) For older pregnant women aged ≥ 35 years, even if the screening result is low risk, the doctor should inform the difference between prenatal screening and prenatal diagnosis and give the pregnant woman the opportunity to choose cytogenetic testing.
(4) Although the incidence of malformations is a few per thousand, it is a 100% disaster for every family that encounters it. Early detection should lead to timely termination of pregnancy. (5) Ultrasound should be recommended for pregnant women at high risk for NTD to rule out neural tube defects.
(6) Screening for low risk only indicates that the likelihood of a particular congenital defect in the fetus is small, but it is not an absolute exclusion.
7. Misleading issues in prenatal screening for trisomy 21.
(1) A fetus with high risk in prenatal screening must have a problem. Prenatal screening is an analysis of risk probability, not a method of confirming the diagnosis of fetal abnormalities. According to our clinical analysis of large samples over the past few years, the rate of fetal chromosomal abnormalities in screening high-risk pregnant women who undergo amniocentesis for amniotic fluid chromosome testing is around 3%.
(2) It is a method to determine whether the newborn is intelligent or not. 21-trisomy screening is mainly to screen for chromosomal abnormalities and can only detect mental retardation caused by chromosomal abnormalities, and cannot detect mental retardation caused by other reasons.
(3) All trisomy 21 can be detected by screening, and there are no fetal abnormalities after prenatal screening. Since the detection rate of screening cannot be 100% and there is a certain rate of missed diagnosis, low risk of screening is not the same as no risk.