The influence of gestational age on Down syndrome screening results

People are not so clear about the gestational age and results of Down syndrome screening, so they are not very clear about the specific conditions of their own bodies, making it impossible to better regulate their bodies. In fact, it is still necessary to do regular examinations as much as possible, especially in the late pregnancy, it will become more critical, because it is time to give birth soon and cannot be ignored.

Currently, the Down syndrome screening test tests the concentration of alpha-fetoprotein (AFP) and human chorionic gonadotropin (β-hCG) in the pregnant woman's blood, and combines it with the pregnant woman's age to accurately calculate the risk of the pregnant woman carrying a fetus with Down syndrome using a computer.

The alpha-fetoprotein (AFP) generally ranges from 0.7 to 2.5 MOM (multiples of the median), and the higher the chorionic gonadotropin, the higher the chance that the fetus will have Down syndrome. In addition, the alpha-fetoprotein value, human chorionic gonadotropin value, and the pregnant woman's age, weight, and weeks of pregnancy can be entered into the computer, which will calculate the risk of the fetus developing Down syndrome. Different hospitals use different standards. If the probability indicated by the test result (such as 1/100) is greater than the normal reference value probability (such as 1/275), the result is positive, indicating that the fetus has a higher chance of being sick and further amniocentesis or chorionic villus sampling should be performed.

1. AFP (alpha-fetoprotein): AFP is a fetal-specific globulin with a molecular weight of 64,000 to 70,000 Daltons. It may have the immunoregulatory function of glycoprotein during pregnancy and can prevent the fetus from being rejected by the mother. AFP is synthesized by the yolk sac in the first 1 to 2 months of pregnancy, and then mainly by the fetal liver. A small amount of AFP can also be synthesized in the fetal digestive tract and enter the fetal blood circulation.

2. The fetal blood AFP value increases rapidly at 6 weeks of pregnancy, reaches a peak at 13 weeks of pregnancy, 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.