INTRAUTERINE DEVICE (IUDs): These are small mechanical devices made up of plastic, copper or stainless steel. They are in the form of loop, coil, T or 7 shaped. IUD is inserted into the uterus of the female by an experienced gynaecologist. It could remain inside the uterus for up to 3 years. IUD prevents implantation of the blastocyst and prevents conception. It is a reliable method of birth control.
There are 3 forms of IUDs: 
i) Non-medicated IUDs: These IUDs phagocytes sperms within the uterus. Ex: Lippes loop.
ii) Copper releasing IUDs: These IUDs release copper ions within the uterus which decrease the sperm motility and suppress the fertilizing capacity of sperms. E.g. Copper-T (CuT), Copper-7 (Cu7), multiload 375 etc.
iii) Hormone releasing IUDs: These IUDs release hormones which make the uterus unsuitable for implantation and cervix hostile for sperms. E.g. Progestasert, LNG-20 (levonorgestrel 20).