The most effective method of prenatal screening for Down syndrome is currently a combined test of maternal serum alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), and free estriol (uE3), combined with age. The three indicators are effectively combined to calculate the individual risk rate for a pregnant woman to produce a newborn with Down’s syndrome. A risk rate greater than or equal to a specific threshold is considered positive. With a detection rate of 75% and a false-positive rate of 4%, it is a convenient and effective screening tool. This article details the details of this method. Down’s syndrome A, free estrogen, human chorionic gonadotropin, and pregnancy-associated protein A Down’s syndrome DS is the most common form of severe congenital mental retardation. The clinical manifestation is severe irreversible mental retardation, with survivors being completely unable to care for themselves. The disease occurs due to chromosome 21 trisomy, and since the cause is still unknown, prior prevention is not possible, but only through prenatal diagnosis and selective abortion to prevent the birth of newborns with DS. Screening facilities have been established in many countries to provide universal screening for women in mid-pregnancy (15-26 weeks). Initially, screening for DS was limited to women over 35 years of age because of the higher incidence in women of advanced age. However, 80% of newborns with DS are born to women under the age of 35. Screening only women over 35 years of age would only reduce the number of DS births by less than 20%. In order to achieve better prevention, a commonly praised screening tool has recently been established. Decreases in maternal blood alpha-fetoprotein (AFP) and free estriol (uE3) in mid-pregnancy and increases in chorionic gonadotropin (hCG) levels have been reported in the literature to be associated with DS and to provide their respective information as independent indicators. Laboratory tests in many countries have demonstrated that the use of these three indicators can greatly improve the detection rate of DS. Firstly, a combined test of serum AFP, uE3 and hCG in pregnant women was implemented and evaluated in combination with age. He tested 77 sera of affected pregnant women and 385 sera of normal pregnant women of the same age and gestational week. Using a risk ratio of ≥1:250, he detected a total of 67% of affected pregnant women, with a false positive rate of only 5%. These results were subsequently confirmed by numerous authors. This test is considered to be the most effective screening measure available, and it is worth promoting because of its simplicity, cost savings, and the fact that most pregnant women avoid the risks associated with amniocentesis. Before the start of the combined screening test, some basic data should be obtained, namely the normal median values of the three indicators AFP, uE3 and hCG at each week of pregnancy (15-26 weeks). The weekly values are derived from values measured in no less than 100 normal pregnant women. This rule only covers midterm (15-26 weeks) singleton pregnancies. The feasibility of this test in early pregnancy has been debated. Some scholars have found that hCG, the most meaningful diagnostic indicator for DS, is relatively insensitive at this stage, and Brizot et al. also found no statistical difference between the AFP values of the affected and normal groups in early pregnancy, so they pointed out that the combined screening method should not be performed in early pregnancy. In contrast, a number of recent studies have shown that the serum free β-gCG levels of affected mothers in early pregnancy are much higher than those of normal pregnant women, and the levels of AFP and uE3 in the blood of affected mothers are also significantly lower in this period, so these scholars speculate that the test can also be applied in early pregnancy. The maternal venous blood is drawn at 15-26 weeks of gestation and serum is retained. Since the assessment of the three indicators is based on the week of gestation, and a two-week difference in the week of gestation will result in a nearly 10% difference in the risk rate for the assessment of the three indicators, the accuracy of the week of gestation is crucial. Gestational week is generally determined based on last menstrual period projection or biparietal diameter (BPD) by ultrasound. Routine use of ultrasound can increase the detection rate by 8-10% while keeping the false positive rate constant. The results of all three assays are expressed as median multiples (MOM), MOM = assay value/median value (of the assay). (i) AFP test Since the AFP value in maternal blood is affected by many factors such as maternal weight, race and history of diabetes mellitus, it needs to be corrected. 1.Correction of weight Since the AFP level in maternal blood decreases with the increase of maternal weight, it should be corrected as follows: Corrected value MOM = measured value (MOM)/10[0,2658-0,00188×weight (pounds)] The applicable range is 90-280 pounds, below 90 pounds is calculated according to 90, above 280 pounds is calculated according to 280. 2, corrected for racial factors Black normal pregnant women had 15% higher AFP values than white normal pregnant women, and the median value of affected black pregnant women was also significantly higher than affected white pregnant women (41%), for reasons unknown. In the absence of the median value of this indicator in normal black women, it is appropriate to multiply the test value of white women by 1.15 or divide the test value of black women by 1.15 to correct for 3. The difference is most significant before 21 weeks. The difference in AFP levels between pregnant women with diabetes mellitus and normal pregnant women at the beginning of the second trimester (16-18 weeks) is considered by some authors to be small. The magnitude of the effect of diabetic factors remains to be explored. After the above correction, a more realistic median AFP value can be obtained. (ii) Detection of hCG Some studies have found that, similar to AFP, the level of hCG in the blood of pregnant women also decreases with increasing maternal weight and therefore requires correction. The correction value = measured MOM/e｛0.5145-[0.00372×weight (lbs)]} (iii) Detection of uE3 Pregnant women with combined diabetes have significantly lower levels of uE3 than normal pregnant women. Therefore, it has been suggested that screening facilities should establish median values for the population of pregnant women with comorbid diabetes mellitus. (a) Calculate the age-related risk rate: for example, the asr of a 35-year-old pregnant woman is 1:384 by substituting the formula. (b) Calculate the likelihood ratio: first, we need to assess the degree of correlation between the variables of interest. If the two variables are perfectly correlated, they need not be compared in assessing the hazard ratio; if they are not correlated at all, then each variable provides its own independent information; if they are partially correlated, then each variable may provide some hazard information. Studies have shown that hCG is correlated with uE3 and AFP is weakly negatively correlated with age. Neglecting any of these indicators when performing a comprehensive evaluation will result in the loss of some information. Using the table as an example, if a 35-year-old pregnant woman has an AFP of 0.4 MOM and a uE3 of 0.4 MOM, the significance of the hazard ratio is evident when the hCG is 0.5 MOM, which is 1:370, and when the hCG is 1.0 MOM, which is 1:84. (iii) Calculate the hazard ratio. (iv) Results Judgment is generally based on a risk rate ≥ 1:274 (the risk rate of fetal involvement of DS in mid-pregnancy in 35-year-old pregnant women) as a positive screening test. If the risk rate is not reached in 35-year-old pregnant women, it can be treated as negative; conversely, if such a risk rate is reached in pregnant women below 35 years of age at screening, it should be treated as a positive screening test. Cuckle noted that repeated measurements with a tendency to increase in AFP should not be treated as negative, even if the second measurement is not positive. Therefore, he believes that repeated measurements will only delay the test and are not advisable. If a pregnant woman has not had an ultrasound prior to this test, it is recommended that those with positive results receive an ultrasound diagnosis. The first step is to exclude conditions such as multiple births, stillbirths or premature miscarriage, and then to further determine the correct gestational week. If the gestational week is calculated accurately, the woman should be included in the high-risk group; if the gestational week is calculated accurately, the woman should be included in the high-risk group; if the gestational week is not calculated correctly (difference >d), the woman should be re-evaluated using the exact gestational week; if the gestational week is <15 weeks, the woman should come back at 16-18 weeks to take a blood sample for testing. Any finalized positives are high-risk pregnant women and genetic testing (amniocentesis or chorionic villus biopsy) is recommended to determine karyotype. The above details are summarized in the procedure chart. It is important to emphasize that although this screening method is generally accepted, there is a growing number of scholars working on new, more sensitive and effective methods, and some progress has been made. The use of ultrasound testing as an adjunct has been reported to potentially improve detection rates. The detection of pregnancy-associated plasma protein A (PAPP-A) and pregnancy protein 1 (sp1) in maternal blood in early pregnancy has been reported to be useful for the early diagnosis of DS. The gestational age of the mother does not affect the detection rate of DS. Recently, a 63% detection rate of DS at 9-13 weeks of gestation was reported using a combination of maternal age and pregnancy-associated protein A and hCG. This is a meaningful report for screening in the first trimester of pregnancy.