The IUD is placed in the woman’s uterus to prevent pregnancy in two ways: First, the IUD prevents the union of sperm and egg. After intercourse, sperm will pass through the vagina, cervix and uterine cavity to the fallopian tubes to meet the egg to form a fertilized egg. If there is an IUD in the uterine cavity, it will be difficult for the sperm to cross the uterine cavity into the fallopian tube after entering the uterine cavity through the cervix, so it can achieve the effect of contraception. Secondly, even if there is a strong sperm, it can enter the fallopian tube through the uterine cavity and meet the egg, the fertilized egg still needs to be transported through the fallopian tube to the uterine cavity to be laid and continue to grow.