Let’s start with the process of sperm-egg union, when ovulation occurs, the mature follicle ruptures, and the secondary oocyte, together with the surrounding zona pellucida and the radial crown, is discharged from the ovary, and the egg is picked up by the umbilical end of the fallopian tube within 2~3 minutes after ovulation. Due to the oscillation of the cilia of the fallopian tube epithelial cells and the contraction of the muscularis propria, the oocyte is rapidly transferred to the tubal pudendum. For human mature sperm, when ejaculating, sperm must pass through the total length of 20~40cm of the male and female reproductive tract to reach the tubal abdomen, and less than one in a million sperm are able to finalize this journey. Each ejaculation of about 200-500 million spermatozoa enter the vagina, some of them flow out of the vagina with semen, the rest of the spermatozoa rely on the tail of the swing swim, which forward movement of spermatozoa, you need to pass through the barrier of cervical mucus and phagocytosis of leukocytes in the uterine cavity in turn, the final spermatozoa that can enter the fallopian tube is usually only a few hundred. The process by which sperm gains the ability to fertilize after a period of incubation in the female reproductive tract is called sperm capacitation. After sperm capacitation, the acrosome reaction can occur, secreting acrosomal enzymes in order to dissolve the radiolucent crown and zona pellucida around the oocyte. Not the first sperm that reaches the fallopian tube and meets the oocyte can complete fertilization, the group of sperm that reaches the fallopian tube needs to surround the oocyte to secrete acrosomal enzymes, which together open the way for the sperm to enter the oocyte, and ultimately, only the most fortunate one of the sperm can enter the oocyte to complete fertilization, and thereafter, the ability of the zona pellucida to bind to the sperm decreases in order to prevent the occurrence of polytrichous fertilization. Therefore, only a certain number of forward-moving sperm can complete the fertilization process. Artificial insemination refers to the injection of washed and processed male semen into the female reproductive tract by non-coital artificial means in order to fertilize the egg and sperm naturally for the purpose of pregnancy. Our center uses intrauterine insemination (IUI), which avoids the loss of sperm at the vagina and cervix, and promotes sperm capacitation and improves fertilization by washing the sperm. However, IUI is the natural fertilization of sperm and egg, which still requires a certain number of sperm to swim from the uterine cavity to the fallopian tubes to complete the fertilization process. Therefore, IUI is only applicable to patients with normal semen, mild oligo-weak semen, and semen that is not liquefied or incompletely liquefied. For patients with moderate or severe oligozoospermia, the number and density of sperms after sperm washing may still not be able to meet the requirements of natural fertilization, and they need to rely on in-vitro fertilization as a means of assisted conception.