The exact mechanism of the copper IUD is still not completely understood. The most recent and widely accepted theory is that it prevents fertilization by altering the environment within the uterus and fallopian tubes so that it is hostile to sperm.
The IUD was once thought to prevent implantation (i.e., the sperm and ovum unite and begin dividing but the resulting embryo does not become embedded in the uterine lining). This earlier theory appears to be accurate in only a very small minority of cases.
The IUS works like other progesterone-only contraceptives by suppressing ovulation and altering the cervical mucus, thus preventing fertilization. In addition, as above, the presence of a foreign body in the uterus seems to interfere with both fertilization and implantation. All types of IUS release the synthetic progesterone levonorgestrel, although at different dosages.
A physician inserts the IUD or IUS into the uterus through a narrow applicator tube, which is generally less than 10 mm in diameter. The IUD/IUS opens to its full shape inside the uterus. When open, the IUD/IUS is wider than the opening of the uterus and thus should remain in place until medically removed.
After insertion, the IUD/IUS requires no further action on the part of the couple. One or two small strings are left protruding from the cervix to allow confirmation that the device has not spontaneously slipped out. These strings also facilitate removal when desired.