Before we learn about amniocentesis, we need to learn about chromosomes: chromosomal DNA is the carrier of genetic units that determine the genetic information of each of us.

The sperm and the egg each contains 23 chromosomes. After fertilization, the chromosomes in the egg and sperm cells combine to form a zygote with 46 chromosomes, from which all cells of a new life divide. Chromosomal diseases occur when abnormalities in chromosome number and structure lead to an increase or decrease in genetic material.

Down's syndrome, commonly known as trisomy 21, is a common chromosomal disorder. It is based on 46 chromosomes and has an additional chromosome 21, which can lead to severe intellectual disability. In addition, there are some chromosomal diseases characterized by small fragment deletions or duplications, or monogenic lesions on the chromosome.

1. So, for pregnant women, what should be the choice of Down's screening, DNA-based noninvasive prenatal testing, amniocentesis?

There are currently three methods for prenatal examination of Down's syndrome: Down's screening; DNA-based noninvasive prenatal testing; and amniocentesis.

Down's screening: some biochemical indicators in maternal blood are detected by biochemical methods and the risk of fetal trisomy 21, trisomy 13, trisomy 18 is predicted by combining the age, weight, gestational week, ultrasound examination of pregnant women, etc.

DNA-based noninvasive prenatal testing: the principle of the detection is based on the free DNA of the fetus in the maternal blood. By collecting maternal blood and using high-throughput sequencing technology to sequence the free DNA fragments in maternal blood, the risk rate of fetal trisomy 21, trisomy 13, trisomy 18 syndrome is obtained.

Amniocentesis: the fetal cells or fetal DNA obtained from the amniotic fluid are genetically examined.

The first two methods are for screening and amniocentesis is for diagnostic.

Accuracy comparison:

Down's screening<DNA-based noninvasive prenatal testing<Amniocentesis

Comparison of examination contents' width:

Down's screening<DNA-based noninvasive prenatal testing<Amniocentesis

Safety factor comparison:

(Down's screening=DNA-based noninvasive prenatal testing)>Amniocentesis

Cost comparison:

Down's screening<DNA-based noninvasive prenatal testing<Amniocentesis

According to established principles, in the absence of high-risk factors for chromosomal diseases, Down's screening can be selected first and if there are abnormalities, the other two tests can be considered.

If economic conditions permit, direct selection of DNA-based noninvasive prenatal testing may also be considered in the absence of high-risk factors. However, if the results show that the risk of chromosomal disease in the fetus is low, Down's syndrome still cannot be ruled out 100 percent -- and if it is high, amniocentesis is needed to confirm the diagnosis.

If at the time of prenatal diagnosis, the doctor informs the patient that there are high-risk factors, amniocentesis can be directly selected.

2. If prenatal ultrasonography shows the fetus to be in normal condition, why should amniocentesis be performed?

Prenatal ultrasounds cannot exclude all chromosomal abnormalities and amniocentesis is still recommended for pregnant women with high risk factors for chromosomal diseases. Ultrasonography can only look at the structure and growth and development of the fetus.

3. I only had Down's screening before I gave birth to my first baby. Why did I have amniocentesis when I was pregnant with my second baby?

The sperms and eggs are different at each fertilization. As pregnant women grow older, the age of the eggs will also increase. Older eggs and sperms are prone to abnormal fertilization when they are fertilized and the probability of remaining babies with chromosomal abnormalities will increase. Therefore, more caution should be taken when conceiving a second baby.

4. There is no genetic disease in our family. My husband and I have normal chromosomes. Why should we check the baby's chromosomes?

Not every combination of sperm and egg is perfect. If there is a problem in the number of chromosomes or genetic variations in the process of sperm and egg production or when they are combined, the fertilized egg -- which is the product of sperm and egg combination -- will also carry the wrong genetic information, thus leading to chromosomal abnormalities in the baby.

5. Does it hurt when receiving amniocentesis? Do I need anesthesia?

According to most pregnant women who have undergone amniocentesis, amniocentesis is the same as a regular injection and some even feel no pain from the injection. No anesthesia is required.

6. Is amniocentesis dangerous? Will amniocentesis stab my baby?

Amniocentesis has a certain risk, but the overall incidence of surgical complications is low. The main complications include infection, bleeding, premature rupture of membranes, fetal loss, etc. Comprehensive statistics show that the risk of fetal loss is 0.5 percent.

Amniocentesis is usually performed under the guidance of ultrasound, which will avoid damage to the fetus as much as possible.

7. The amniocentesis results show normal, then the baby must be healthy?

In terms of current levels of science and technology, our existing methods of chromosome karyotype analysis can help us understand whether the fetal DNA is missing. Chromosome microarray analysis (CMA) detection methods can help us understand whether there are more or less segments of DNA fragments. Our single gene detection methods can help us understand whether there are errors in the fetal DNA fragments.

However, little is known about the diagnosis of monogenic diseases, so we cannot tell you that your child must be healthy at the current levels of science and technology.

8. How much does amniocentesis cost? How long will the results be known after the examination?

According to the different detection methods and content, the price generally ranges from 2,000 yuan ($283.41) to 10,000 yuan, most of which are around 2,000 to 5,000 yuan, and the time to produce the results ranges from a few days to more than a month.

9. Doctor, I faint when I have a needle, how do I receive an amniocentesis?

Just close your eyes.