What are the methods of prenatal diagnosis?

Many prenatal diagnosis methods are unfamiliar during pregnancy. Once a baby disease occurs, it will cause very serious harm and even affect the child's life. Therefore, prenatal diagnosis must be performed during pregnancy. First, professional examinations are needed to detect problems as early as possible. So, what are the methods of prenatal diagnosis?

1. Prenatal diagnosis methods

Prenatal diagnosis methods can be divided into three categories and five levels

(I) The first type is to use special instruments to check whether there are any deformities on the fetal surface, such as X-rays or surface angiography, indirect observation by B-type ultrasound scanning; or direct observation under a fetal microscope. This type of examination is at the morphological level.

(ii) The second type is to use special tests such as maternal blood and urine to indirectly diagnose congenital diseases in the fetus. During pregnancy, a small amount of fetal blood cells, diffusible metabolites, proteins, and enzymes can enter the maternal blood circulation through the placenta. This is the basis for prenatal diagnosis of certain diseases using maternal blood and urine. For example, measuring maternal blood alpha-fetoprotein (AFP) to diagnose fetal neural tube defects (NTDs), and measuring pregnant women's urine methylmalonic acid to diagnose fetal methylmalonic aciduria.

(III) The third category is to directly obtain fetal blood, amniotic fluid or fetal tissue to diagnose fetal diseases.

Three types of examination methods can perform prenatal diagnosis at five levels: morphology, chromosomes, enzymology, metabolites and genes.

2. Prenatal diagnosis of common congenital diseases

Among congenital diseases, the more common ones are chromosomal diseases, neural tube defects and metabolic genetic diseases. Clinically, it manifests as developmental malformations, intrauterine death of the embryo or fetus, leading to miscarriage, premature birth, stillbirth, or neonatal death. The survivors show various deformities, functional disorders, and intellectual disabilities. If congenital diseases can be diagnosed prenatally, the birth of affected children can be prevented, which will be of great benefit to the family and society.

1. Prenatal diagnosis of neural tube defects

Neural tube defects (NTDs) refer to a group of central nervous system malformations caused by impaired neural tube closure during the fetal period or re-perforation of the neural tube after closure due to other reasons, including anencephaly, open spina bifida and encephalocele. The incidence of NTD in my country is 0.66-10.53‰, with an average of 2.74‰. It ranks first among birth defects in my country and has been listed as a key research topic by the country.

1. The maternal blood AFP measurement is used as a preliminary screening. If the maternal blood AFP is greater than 2 standard deviations of the normal level of pregnant women in the same period, the examination will be repeated. If it is still significantly elevated, the amniotic fluid AFP measurement will be performed.

2. Amniocentesis is performed between 16 and 20 weeks of pregnancy to measure the AFP content in the amniotic fluid. If it exceeds the normal value by 3 to 5 standard deviations, the diagnosis of NTD can be established. About 90% of NTDs can be diagnosed through AFP measurement.

3. Amniotic fluid acetylcholinesterase (AChE) assay AchE is produced in the nervous tissue and can penetrate into the amniotic fluid during NTD, resulting in a significant increase in AchE activity in the amniotic fluid. The content of this enzyme is relatively stable, is not affected by pregnancy or fetal blood contamination, and makes up for the shortcomings of amniotic fluid AFP determination.

4. Ultrasound examination is performed in the second trimester. The ultrasound image characteristics of anencephaly are: ① lack of skull halo; ② the fetal head is replaced by a "tumor"-like object; ③ the orbital and nasal bones are visible on the "tumor"; ④ the meningeal sac is visible behind the "tumor"; ⑤ often accompanied by spina bifida and polyhydramnios.

5. Examinations such as abdominal X-ray films and amniocentesis can also be used. But it is rarely used now.

2. Prenatal diagnosis of chromosomal diseases

Most chromosomal diseases result in miscarriage, so they only account for about 5% of the total births, but the diagnosis rate is relatively high, accounting for 25% to 50% of the cases diagnosed prenatally.

Target: 1. Pregnant women over 35 years old; 2. Have given birth to a child with chromosomal disease; 3. One of the couple is a carrier of chromosomal translocation; 4. Those who have given birth to children with NTD; 5. Pregnant women with multiple miscarriages, stillbirths or fetal death with unexplained causes; 6. The couple has congenital metabolic disease or has given birth to a child with metabolic disease; 7. Those who have serious sex-linked genetic diseases in the family; 8. Long-term exposure to substances that are harmful to pregnant women and fetuses (such as radiation and pesticides).

Diagnostic methods: Early chorionic villus direct preparation, amniotic fluid cell culture, maternal blood and fetal blood cells, etc., can be used for chromosome karyotype analysis to confirm the diagnosis. In units with the conditions, new technologies such as DNA recombination, DNA gene amplification (PCR), and gene analysis can be used for diagnosis.

3. Prenatal diagnosis of metabolic genetic diseases

Metabolic genetic diseases are caused by mutations in genes on chromosomes, which result in enzyme deficiency or abnormality. The catalytic process of a certain enzyme controlled by the original gene cannot proceed normally, causing the metabolic process to become disordered and damaged, resulting in the lack of some substances and the accumulation of other substances, thus affecting the metabolism and development of the fetus. More than 1,000 diseases have been discovered so far, most of which are autosomal recessive, and a few are X-linked recessive and autosomal dominant.

Diagnostic methods: 1. Check the pregnant woman's blood or urine for specific metabolites, such as measuring methylmalonic acid in urine; 2. Amniotic fluid analysis, to measure abnormal metabolites released by the fetus in the amniotic fluid, such as the content of 17-ketosteroids in adrenogenital syndrome; 3. Under the guidance of B-ultrasound or fetoscopy, fetal blood, chorionic villus cells, amniotic fluid cells, etc. are cultured and enzymes or other biochemical components are measured for diagnosis. New technologies such as DNA recombination and DNA amplification enzyme-linked polymerase reaction (PCR) can also be used.

The above introduces the prenatal diagnosis methods. I believe everyone should have a clear understanding of them. What we need to remind you here is that pregnant mothers need to undergo relevant examinations before delivery, and once problems are found, timely measures should be taken to prevent fetal diseases from causing harm to the child and the family.