Prenatal diagnosis is a means of purposefully using various methods to diagnose whether a fetus is suffering from a certain genetic disease or congenital malformation before birth on the basis of genetic counseling. The so-called genetic diseases are diseases or defects caused by changes in the genetic basis, which include the following three types. (a) monogenic genetic diseases: monogenic genetic diseases are typical Mendelian inheritance, can be divided into the following types. 1, autosomal dominant genetic diseases: this disease is the formation of dominant mutations in the genes on the autosomes, such as familial hypercholesterolemia, polydactyly, syndactyly, congenital chorea, atrophic ankylosis, periodic migraine, multiple familial colon polyps, chondrodysplasia, etc.. It is characterized by a dominant gene and can develop when the gene is heterozygous, with no gender restriction. In the genealogy, it is often passed from generation to generation, and the causative gene is often passed to the offspring with a 50% or 100% chance. Of course, not all dominant genetic disorders manifest immediately after birth, such as congenital chorea, which is often caused by a decline in some central areas of the brain controlled by a dominant single gene in adulthood, resulting in involuntary tremors of the face, body, and limbs. 2, autosomal recessive disorders: This disease is formed by recessive mutations in genes, such as congenital deafness, galactosemia, gastric ulcer, melanuria, phenylketonuria, lipid-like deposition, congenital idiocy, high myopia, albinism, etc.. The genetic characteristics are that the disease develops only when the causative gene is in the purely heterozygous state and the heterozygous state is normal. Therefore, in the genealogy, the disease is generally not continuous, sporadic and often inherited intergenerationally. The likelihood of carrying recessive genes in a population is high, and generally conquer individuals with several recessive genes in their bodies, and in random marriages, the incidence of offspring is low because the chances of both spouses carrying the same disease causing gene are small. In case of consanguineous marriages, the possibility of the couple carrying the same disease-causing gene is very high, and the incidence of genetic diseases in their offspring is also significantly increased. This is the main scientific basis for the prohibition of consanguineous marriage. 3, X chromosome dominant genetic disease: This disease is a hereditary disease controlled by the dominant pathogenic gene located on the X chromosome, such as: anti-vitamin D rickets, the disease patients due to phosphorus, calcium malabsorption and lead to bone development disorders. Patients often show symptoms such as X-shaped (or O-shaped) legs, deformed bone development (such as chicken breast), and slow growth. 4, X chromosome recessive genetic disease: This is a genetic disease controlled by the recessive pathogenic gene located on the X chromosome, such as: progressive muscular dystrophy (pseudohypertrophy), the affected child due to muscle atrophy, weakness and walking difficulties, the late stage of the disease bilateral gastrocnemius muscle is pseudohypertrophy (muscle tissue is replaced by connective tissue). Most children develop the disease at the age of 4 to 5 years and die before the age of 20 years. (B) chromosomal genetic diseases: This disease is caused by chromosomal number or structural aberrations, the incidence of about 0.5% in newborns, can be divided into two types of autosomal and sex chromosomal disorders. 1, autosomal disease: autosomal number abnormalities are more common, manifested as an extra autosomal number, called trisomic type. Individuals with trisomic phenotypes often have morphological features, the nature and extent of which depends on which chromosome number is in the trisomic state. Structural abnormalities of autosomes are mostly common with deletions, duplications, inversions and translocations. Fetuses with autosomal abnormalities are more likely to be aborted and, if born, to have multiple malformations, mental retardation and developmental disorders. Common autosomal disorders include trisomy 21 (congenital stupidity), trisomy 18 (Edward’s syndrome), trisomy 13 (Patau’s syndrome) and catcalling syndrome (5P-syndrome). 2. Sex chromosome disorders: These are genetic disorders caused by sex chromosome variations, such as gonadal dysgenesis (also known as Turner syndrome). The incidence of gonadal dysgenesis, the most common sex chromosome disorder in women, is 1/3500, and chromosomal examination reveals that the patient is missing an X chromosome. Patients with this disorder have a female appearance, but have hypogonadism, no breast development, and therefore no fertility. About 30% of patients with this disease have congenital heart disease. (iii) Polygenic genetic disorders This disorder is determined by multiple pairs of genes in concert and is influenced by the environment, such as: hypertension, diabetes, asthma, schizophrenia, etc. Each pair of genes is not dominant or recessive to each other and is influenced to a greater extent by environmental factors. In polygenic diseases, the more pairs of genes involved in trait determination, the more types of expression and the smaller the differences between types. The closer the blood relationship, the higher the incidence. Second, what are the methods of prevention and treatment of genetic diseases (a) diagnosis of genetic diseases 1, genealogical analysis: genealogical analysis is based on the medical history of the family members of the patient genealogical mapping, and through the analysis of the genealogy to determine the disease genetic pathway of a diagnostic method. If the patient suffers from a single disease symptom and shows vertical transmission, the disease is likely to be a genetic disease, the diagnosis can be used genealogical analysis. Genealogical analysis can diagnose whether the disease is a genetic disease on the one hand, and further determine whether the genetic disease is monogenic or polygenic on the other. 2. Chromosome examination: The karyotype of the patient is analyzed to determine whether the chromosomes of the patient have aberrations. There are two types of karyotype analysis: dominant band karyotype analysis and non-dominant band karyotype analysis. The non-dominant karyotype analysis does not completely reveal the morphology and characteristics of the chromosomes, but it can identify the specific number of chromosomes, which can determine whether the patient has chromosomal number aberrations and which chromosomes the aberrations occur on, and the results are expressed in a karyotype chart with the diagnosis, e.g., 47, XY, +21 for congenital dysgenesis – trisomy 21. The karyotype analysis of the chromosomes is done by banding technique, and the chromosomes are analyzed to find out where the aberrant chromosomes are located, and the results are expressed in a karyotype chart with the diagnosis, e.g. 46, XX (XY), 5 P – indicates that the short arm of one of chromosome 5 is missing. Prenatal diagnosis: Prenatal diagnosis is a means of purposefully using various methods to diagnose whether a fetus is suffering from a certain genetic disease or congenital malformation before birth on the basis of genetic counseling. The application of prenatal diagnosis technology, before the birth of the fetus, especially in the early stages of pregnancy, the embryo or fetus can be properly examined, early understanding of the normal development of the fetus. If the fetus is confirmed to be normal, it can relieve the psychological burden of the pregnant woman and her family, which is conducive to pregnancy care; if the fetus is abnormal, the diagnosis will be made after obtaining the analysis data, and then select the measures to stop the pregnancy or intrauterine treatment, in order to achieve the purpose of reducing the birth of children with genetic diseases and malformations. Genetic diagnosis: Genetic diagnosis can not only clearly point out whether an individual has a disease, a genetic defect and reveal its genetic status, but also make diagnosis and prediction for phenotypically normal carriers and susceptible persons of certain diseases. Compared with traditional diagnostic methods, genetic diagnosis has many features, namely, diagnosis for direct etiology; high specificity and sensitivity; adaptability and wide diagnostic range; and no tissue and developmental specificity of the target gene. Therefore, genetic diagnosis can be generally used for pre-symptomatic diagnosis, prenatal diagnosis and carrier detection in clinical applications. (B) Prevention of genetic diseases 1. Careful choice of spouse: Avoid marrying each other with people suffering from the same kind of genetic diseases, because the chance of their offspring suffering from the same kind of genetic diseases as their parents will be significantly increased when they marry each other. Such diseases as congenital asthma, primary hypertension, diabetes, atherosclerosis, cleft lip, congenital deafness and high myopia. If two patients with primary hypertension are married, the chances of their offspring suffering from primary hypertension will be as high as 47% or more. 2, avoid consanguineous marriage: to avoid consanguineous marriage, this is because the incidence of recessive genetic diseases in the offspring of consanguineous marriage will be significantly higher than the general population. Some research results show that the offspring of patients with a variety of serious diseases, the possibility of hereditary diseases will also increase. 3, to choose the right time to conceive the appropriate age of the couple: the woman more than 35 years old, the offspring can increase the chance of congenital stupidity about 10 times; the male partner’s age should preferably not exceed 50 years old. It is important to pay attention to the “external environment” in which both men and women’s bodies are exposed to at the time of conception. If you are in close contact with toxic or harmful substances (such as receiving radiation treatment or spraying pesticides), or are applying a drug that can cause damage to the embryo, you should not conceive immediately and avoid the harmful external environment for a period of time before conceiving. 4.Prenatal diagnosis: For pregnant women whose one of the spouses is a patient of some genetic disease or has given birth to a child with some genetic disease; pregnant women with unexplained history of multiple miscarriages, abnormal fetuses, stillbirths or neonatal deaths; senior pregnant women over 35 years old; pregnant women with obvious history of exposure to teratogenic factors, such as viral infection in early pregnancy, taking improper drugs, inappropriate exposure to chemical substances, etc., prenatal diagnosis should be conducted in early and middle pregnancy, Prenatal diagnosis should be carried out in the middle of pregnancy to prevent the birth of malformed children or children with genetic diseases.