Infertility is defined as a couple of childbearing age, not using contraception after marriage, having normal sexual intercourse, living together for 2 years and failing to conceive. The time in the World Health Organization’s definition of infertility is 1 year, with the aim of early diagnosis and treatment. Infertility can be further divided into primary and secondary infertility according to whether or not they have suffered from infertility. Primary infertility refers to never having had a pregnancy; secondary infertility refers to having had a pregnancy and then not having another pregnancy without contraception for more than 1 year. Relative infertility can be divided into relative infertility and absolute infertility according to the cause of infertility; relative infertility refers to temporary infertility caused by one of the couple preventing conception or reducing fertility for some reason, and if the factor is corrected, pregnancy is still possible. Infertility is a reproductive disorder caused by many diseases or multiple factors; it is not an independent disease, but a complex clinical syndrome. In recent years, the incidence of infertility has been increasing year by year. According to the data, the incidence of this disease is about 15%-20% of couples of childbearing age in developed countries in the world in recent years, and the incidence of infertility in China is conservatively estimated to be about 8%. This disease has become a global medical and sociological problem that affects human development and health. What are the causes of infertility? Before answering this question it is important to first understand the basic process of pregnancy. Conception is a complex physiological process that requires the following basic conditions: the woman’s ovaries discharge normal eggs; the man produces normal sperm; the egg and sperm can meet in the fallopian tube and unite to form a fertilized egg and the fertilized egg can be transported smoothly to the uterine cavity; and the endometrial conditions are suitable for the fertilized egg to be laid. Only when all these conditions are normal can the basic requirements for conception be met, and dysfunction in any of them will prevent smooth conception. Moreover, pregnancy is a reflection of the reproductive function of both men and women and is not only a female problem. Among the various causes of infertility, female factors account for about 40-55%, male factors account for about 25-40%, factors belonging to both partners account for about 20%, and immune and unknown causes account for about 10%. The specific reasons are as follows: (1) Female infertility factors: (1) abnormal vulvar and vaginal development: hermaphroditism including true hermaphroditism and pseudohermaphroditism; abnormal hymen development: hymenal atresia, hard hymen, etc.; abnormal vaginal development: congenital complete or partial vaginal atresia, double vagina or vaginal longitudinal septum. (2) Scar stenosis: vaginal injury forms adhesive scar stenosis, which affects the entry of sperm into the cervix and affects fertilization. (3) Vaginal inflammation: Severe vaginitis can significantly change the pH of the vagina, with a large number of microorganisms and white blood cells in the vagina. The abnormal environment of the vagina reduces sperm vitality, shortens its survival time, and even engulfs sperm, which affects fertilization. Mycoplasma and Chlamydia can cause pelvic inflammation and lead to weakened cilia movement in the fallopian tubes, resulting in infertility. 2. Cervical factors: The cervix is the way for sperm to enter the uterine cavity, and abnormalities in the location, amount and nature of cervical mucus can affect whether sperm can enter the uterine cavity. (1) Abnormal development of the cervix: congenital cervical stenosis or atresia, poor elimination of menstrual blood in mild cases, reduced menstrual flow, dysmenorrhea, and possible complications of endometriosis. The cervical canal is dysplastic and slender, which affects the passage of sperm; the mucous membrane of the cervical canal is dysplastic then the glandular secretion is insufficient both can lead to infertility. (2) Cervical inflammation: In severe cases, the purulent leucorrhea in the cervical canal increases and becomes sticky, which affects sperm penetration. (3) Cervical redundancy: cervical polyps, cervical fibroids, etc. block the cervical canal and affect fertilization. Uterus factors: (1) congenital malformation of the uterus: abnormal development of the uterus, such as congenital uterine agenesis, stumpy uterus, bicornuate uterus, longitudinal uterus, etc. all affect conception. (2) Endometrial abnormalities: endometritis, endometrial tuberculosis, endometrial polyps, endometrial adhesions or poor endometrial secretion response affect the fertilization of eggs. (3) Uterine tumors: endometrial cancer causes infertility, most patients with endometrial atypical hyperplasia are infertile, uterine fibroids can affect conception, and submucosal fibroids can cause infertility or miscarriage after pregnancy. 4. Fallopian tube factors: The fallopian tube has the function of transporting sperm, collecting eggs and transporting fertilized eggs to the uterine cavity. Tubal pathology is the most common factor in infertility, and any factor that affects the function of the fallopian tubes can affect fertilization. (1) Tubal insufficiency: tubal dysplasia affects peristalsis, which is not conducive to transporting sperm, eggs and fertilized eggs, making it easy for tubal pregnancy to occur; congenital tubal over-elongation and distortion affects the movement of sperm or eggs. (2) Tubal inflammation: Tubal inflammation can cause adhesions at the umbilical end or obstruction of the lumen, and adhesions between the fallopian tubes and surrounding tissues can affect peristalsis and cause infertility. Tubal tuberculosis causes tubal stiffness, fistula, etc. (3) Peritubal lesions: endometriosis is the most common cause of tubal adhesions due to ectopic endometrium forming nodules in the fallopian tubes or ectopic endometrium outside the pelvis. 5. Ovarian factors: (1) abnormal ovarian development: polycystic ovaries, undeveloped ovaries and ovarian insufficiency. (2) Endometriosis: The relationship between endometriosis and infertility is reported to be 41.5% to 43.3% for primary infertility and 46.6% to 47.3% for secondary infertility in patients with endometriosis, while the infertility rate in the normal population is 15%. Severe endometriosis causes adhesions that affect ovarian function and prevent the maturation and release of oocytes. (3) Luteinized unruptured folliclesyndrome (LUFS): Brosen hypothesized that LUFS is one of the causative factors of endometriosis, based on the fact that LUFS patients have fewer 17-beta estradiol and progesterone in the ascites than normal due to unruptured follicles, and lose the inhibitory effect on ectopic endometriotic cells. The patient’s ovaries are not ovulating because of the unruptured follicles and less 17-beta estradiol and progesterone in the ascites than normal. (4) Luteal insufficiency: Insufficient secretion of the luteal phase in ectopic patients affects conception. (6) Ovulation disorders: all factors that cause ovarian dysfunction and lead to ovulation can cause infertility. (1) Central influence: hypothalamic-pituitary-ovarian dysfunction axis disorders, causing menstrual disorders such as anovulatory menstruation and amenorrhea; pituitary tumors causing ovarian dysfunction and infertility; mental factors such as excessive stress and anxiety can affect the hypothalamic-pituitary-ovarian axis and inhibit ovulation. (2) Systemic diseases: severe malnutrition, excessive obesity or lack of certain vitamins in the diet, especially E, A and B, can affect ovarian function; endocrine metabolic diseases such as hyper- or hypothyroidism, hyper- or hypoadrenocorticism, and severe diabetes can also affect ovarian function and lead to infertility. (3) Local ovarian factors: congenital ovarian hypoplasia, polycystic ovary syndrome, premature ovarian failure, functional ovarian tumors such as granulosa-follicular membrane cell tumors and testicularoblastoma can affect ovarian ovulation; ovarian endometriosis not only destroys ovarian tissues, but also causes severe pelvic tissue adhesions and leads to infertility. (B) Male infertility factors Mainly spermatogenic disorders and sperm deferral disorders. External genital and semen examination should be performed to clarify whether there are abnormalities. 1, abnormal semen: such as no sperm or low sperm count, reduced vitality, abnormal morphology, etc. Factors affecting sperm production include: (1) congenital developmental abnormalities: congenital testicular hypoplasia cannot produce sperm; bilateral cryptorchidism leads to atrophy of the varicocele and other obstacles to sperm production can cause infertility. (2) Systemic factors: chronic wasting diseases, such as long-term malnutrition, tuberculosis, chronic poisoning (smoking, alcoholism), and excessive mental stress may affect sperm production. (3) Local causes: mumps complicating orchitis leading to testicular atrophy; testicular tuberculosis destroying testicular tissue. 2, sperm transport obstruction: epididymis and vas deferens tuberculosis can block the vas deferens and prevent sperm from passing; impotence, premature ejaculation can not make sperm enter the female vagina. 3, immune factors: sperm, sperm plasma in the body to produce antibodies against their own sperm can cause male infertility, the ejaculated sperm occurring their own agglutination and can not pass through the cervical mucus. 4, endocrine dysfunction: male endocrine is regulated by the hypothalamus-pituitary-testis axis. Pituitary, thyroid and adrenal gland dysfunction may affect the production of sperm and cause infertility. 5. Sexual function abnormalities: dysplasia of external genitalia or impotence leading to difficulties in sexual intercourse, etc. (3) Factors of both sexes 1. Lack of basic knowledge of sexual life. 2. Excessive mental tension caused by the eagerness of both men and women to have children. 3. Immunological factors: In recent years, research on immunological factors has concluded that there are two kinds of immunological conditions that affect conception. (1) Homoimmunity: sperm, seminal plasma or fertilized eggs are antigenic substances that are absorbed by the vagina and endometrium and produce antibody substances through immune reactions, so that sperm and eggs cannot be combined or fertilized eggs cannot be laid. (2) Autoimmunity: It is believed that the presence of autoantibodies to the zona pellucida in the serum of infertile women prevents sperm from penetrating the egg after reacting with the zona pellucida, thus preventing fertilization. In summary, the causes of infertility are complex and require joint examination and treatment by both men and women. Only detailed and in-depth examination can find the cause of the disease and treat it symptomatically to minimize the treatment time.