Inactive IUCD's are small objects made of polyethylene or nylon. They are thought to inhibit the implantation of the blastocyst by causing chronic irritation of the endometrium. In the absence of symptoms, they may be left indefinitely.
Active IUCD's contain metals or hormones and have largely superseded the passive devices as they tend to be smaller and have lower failure rates.
Copper is commonly used e.g. Copper-7. It is believed to inhibit enzymes present in the endometrium required for implantation. There is no significant absorption systemically and there is no danger from teratogenicity in the case of accidental pregnancy. The copper is slowly lost by elution. Newer devices need replacing every for 4 - 6 years.
The intra-uterine progestogen-only contraceptive releases levonorgestrel directly in to the uterine cavity. This device has a lower incidence of ectopic pregnancy than copper IUCDs and has a failure rate similar to that given by sterilisation.
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