What does amniocentesis check?

Amniocentesis is a prenatal diagnostic method that is more suitable for women in mid-term pregnancy. It uses an amniocentesis ultrasound to pass a long needle through the belly, into the amniotic fluid cavity, and take out a certain amount of amniotic fluid for diagnosis and examination. This can diagnose whether the fetus has malformations, and can identify the child's blood type, fetal lung maturity, placental function and a series of other data.

Amniotic fluid exists in the amniotic cavity. The fertilized egg forms the amniotic cavity and begins to produce amniotic fluid on the seventh day after fertilization. The amount of amniotic fluid is 50 ml at 12 weeks of pregnancy, 400 ml at 20 weeks, and 1000-1500 ml at 36-38 weeks. The amount of amniotic fluid decreases slightly as the expected date of delivery approaches.

The best time to draw amniotic fluid for prenatal diagnosis is between 16 and 24 weeks of pregnancy. Because the fetus is small at this time and there is relatively more amniotic fluid, the fetus floats in the amniotic fluid and is surrounded by a wider amniotic fluid belt. It is not easy to injure the fetus when the amniotic fluid is extracted with a needle. Extracting 20 ml of amniotic fluid only accounts for 1/20 to 1/12 of the total amount of amniotic fluid, which will not cause the uterine cavity to suddenly shrink and cause miscarriage.

Moreover, the proportion of active cells in the amniotic fluid is the largest during this period, and the cell culture survival rate is high. It can be used for slide preparation, staining, fetal chromosome karyotype analysis, chromosome genetic disease diagnosis and gender determination. Amniotic fluid cell DNA can also be used to diagnose genetic and metabolic diseases. By measuring alpha-fetoprotein in amniotic fluid, it is also possible to diagnose open neural tube defects in the fetus.

In the late pregnancy, amniocentesis can measure blood type, bilirubin, lecithin, sphingomyelin, placental lactogen, etc., to understand whether there is maternal-fetal blood type incompatibility, hemolysis, fetal lung maturity, skin maturity, placental function, etc.

The best time to do amniocentesis is between 16 and 20 weeks of gestation, usually for pregnant women aged 35 or above. During the procedure, a needle is inserted into your amniotic fluid under the guidance of a level 2 B-ultrasound to draw out some amniotic fluid, which is then taken out and placed in the laboratory for chromosome culture for about a week to determine whether the baby has any deformities, including cleft lip and congenital diseases. The gender can also be determined.

This experiment is risky, but the accuracy rate is over 99%. The accuracy of Down syndrome screening is about 70%, and it is usually performed between 15 and 20 weeks of pregnancy. The best time to do a level 2 B-ultrasound is at 18 weeks, as it can detect skeletal and visceral deformities including cleft lip, and can also determine the baby’s gender.