The hormonal coil (also known as hormonal IUD) is a small plastic device with some of the hormone progestogen round the shaft. The hormonal coil can sometimes stop ovulation, but it also affects the mucus in the cervix, making it more difficult for sperm to enter the womb. It also thins the lining of the womb, so that a fertilised egg can’t attach to the lining. In this case the effect takes place after fertilisation, causing the fertilised egg to be rejected; this also happens when using the copper coil. We do not recommend using contraceptives that work after fertilisation, even though this occurs very early in the pregnancy.