How to read Down syndrome screening results

Down syndrome screening is actually a screening method to understand whether the fetus has some neural tube defects or other congenital diseases, so you should pay attention to learning how to read the screening results, but usually the doctor will explain the results to the pregnant woman.

(1) AFP (alpha-fetoprotein)

AFP is a fetal-specific globulin with a molecular weight of 64,000-70,000 Daltons. It may have the immunoregulatory function of glycoprotein during pregnancy and prevent the fetus from being rejected by the mother.

AFP is synthesized by the yolk sac in the first 1-2 months of pregnancy, and then mainly synthesized by the fetal liver. A small amount of AFP can also be synthesized in the fetal digestive tract and enter the fetal blood circulation. The fetal blood AFP value increases rapidly at 6 weeks of gestation, reaches a peak at 13 weeks of gestation, and then gradually decreases as the pregnancy progresses to full term. The AFP in amniotic fluid mainly comes from fetal urine, and its change trend is similar to that of fetal blood AFP. Maternal blood AFP comes from amniotic fluid and fetal blood, but its change trend is not consistent with that of amniotic fluid and fetal blood. In early pregnancy, the maternal blood AFP concentration is the lowest, gradually increases as the pregnancy progresses, reaches a peak at 28-32 weeks of pregnancy, and then decreases.

The serum AFP level of pregnant women carrying congenitally retarded fetuses is 70% of that of normal pregnant women, that is, the average MoM value is 0.7-0.8MoM.

(2) Free hCGβ (free β subunit-human chorionic gonadotropin)

Pregnant women with congenitally retarded fetuses have a tonic increase in serum Free hCGβ levels, with an average MoM value of 2.3-2.4MoM. In fact, you don’t need to be too nervous if the MOM value of free-hcg is a bit high.

About: hCG is human chorionic gonadotropin synthesized by placental cells and is composed of two subunits, a- and b-. HCG exists in two forms, intact hCG and the b-chain alone. Both types of hCG are active, but only the b-single-chain form of hCG is the specific molecule for measurement. HCG enters the maternal blood after fertilization and proliferates rapidly until the 8th week of pregnancy, then slowly decreases in concentration until the 18th to 20th week, and then remains stable.

The MOM value is a ratio, that is, the marker detection value in the pregnant woman's body divided by the median value of a normal pregnant woman at the same gestational age. This value is the MOM. Because the levels of prenatal screening substances vary greatly with increasing gestational age, their values ​​must be converted into multiples of the median (MOM) to make them "standardized" and facilitate clinical judgment.

For example: The free-HCG value of a random pregnant woman with a gestational age of 14 weeks + 0 days is: 28800mIU/ml

The median value for gestational age of 14 weeks + 0 days is: 14400mIU/ml

The MOM of this pregnant woman is: 28800/14400=2, so if this indicator alone fluctuates, don't worry too much. It may also be caused by inaccurate calculation of the pregnancy time. There is really no need to panic.

(3) Questions about trisomy 21, 18, and 13

Under normal circumstances, humans have 46 chromosomes in 23 pairs. Trisomy 21, 18, and 13 means that the fetus's 21st, 18th, and 13th pairs of chromosomes have one more than the normal two, which is called XX trisomy. Trisomy 21 is Down syndrome.

Pregnant women of any age may have a fetus with chromosomal abnormalities, but the incidence of chromosomal abnormalities increases significantly with the age of the pregnant woman. For example, the incidence of chromosomal abnormalities in pregnant women under 25 years old is 1:1185, while at the age of 35 it is as high as 1:335. Therefore, pregnant women over 35 years old need to undergo chromosome examination.

1. Down syndrome screening is a possibility test: the high-risk group only means that the fetus is more likely to be a child with Down syndrome, and the low-risk group may also have Down syndrome.

2. About 1/10 of all pregnant women are high-risk groups, and 1-2/100 of the high-risk groups are Down syndrome babies, which means that 1-2/1000 pregnant women are Down syndrome babies.