IUCD - How it works
The conventional IUCD contains copper that is coiled on a T-shaped plastic device. This coil releases small amounts of copper within the cavity of the womb. These particles of copper are hostile to semen thus ensuring that no viable sperm reaches the egg. It usually does not affect the pattern of menstrual flow, except for a slight increase in flow volume.
The Mirena system contains progesterone hormone that is released in small quantities over a period of time, up to a maximum of five years. The hormone thins the lining of the uterus making it unfavourable for implantation to take place. It is thought that the hormone may also reduce the chance of the egg being produced.
Attribution: Avallain