1. The woman’s ovaries have normal mature eggs each month: women of childbearing age have two ovaries, which discharge one egg each month. The egg is discharged into the pelvic cavity and picked up by the umbilical end of the fallopian tube into the fallopian tube. If the menstrual cycle is 28 days long, ovulation occurs on the 14th-15th day of the menstrual cycle, i.e. 14 days before the next menstrual period, and the egg survives for about 16-24 hours. 2. The ovaries can secrete hormones necessary for the maintenance of the oocyte and embryo growth: the development of the egg requires the nutrition of estrogen secreted by the ovaries, and the corpus luteum formed by the ovaries after ovulation can secrete progesterone, which is essential for the maintenance of early pregnancy. 3. The semen of the male partner must be normal: the number, motility and morphology of the sperm in the semen must be normal, other factors such as long liquefaction time, antibodies and white blood cells in the semen are also factors that affect conception. Sperm can survive for 48 hours after discharge, and sexual intercourse should be arranged around the time of ovulation. 4. The woman’s cervix is normal: the cervical mucus becomes clear during ovulation so that the sperm can drill into the cervical mucus and be stored in the cervical canal, released in batches, and swim to the uterine cavity. If there is inflammation in the cervix, the cervical mucus is very sticky and sperm cannot easily enter. 5. The woman’s fallopian tubes are open and functioning normally, so that sperm and egg can meet and fertilize in the tubes and transport the embryo to the uterine cavity: the umbrella end of the tubes can collect the eggs in the pelvic fluid, and the sperm and egg combine in the abdomen of the fallopian tubes. The fertilized egg splits into multiple cells on one side and runs along the tubes toward the uterine cavity for about 3 days. 6. The embryo can be planted in the endometrium: the endometrium thickens after ovulation and has a change of secretion phase. The embryo is planted into the endometrium 3 days after entering the uterine cavity, which is called implantation. Whether the embryo can continue to develop and grow at this time depends on the embryo’s own ability to live and the endometrium secretes enough nutrients. If there is inflammation or damage to the endometrium, the endometrium is like poor soil and the embryo cannot be planted. Infertility can occur if any of the above is impaired. It seems that the causes of infertility can exist in both men and women, or only in either of them.