Correction:
Strictly speaking the coil is not a contraceptive. 'Contraceptive' means to 'prevent conception' - i.e. to prevent the sperm cell fertilising the egg. This happens in barrier methods (condom, diaphragm) and in methods that prevent the egg being released in the first place (the pill), or in methods that cause the sperm or egg to die (as in spermicides), or in methods that prevent sperm and egg meeting through e.g. timing (as in the rhythm method, the method used with the blessing of the Roman Catholic Church).
However, the coil does not prevent conception - it allows conception to take place but prevents the embryo from implanting in the side wall of the womb. Therefore the coil strictly speaking is abortive rather than contraceptive, as it causes the abortion of a otherwise healthy embryo that could go on to develop into a normal child. The 'morning after' pill is another abortive - but unlike the coil that performs is abortive work physically, the morning-after pill does so chemically. The confusion between abortive and contraceptive is seen in the above answer. The writer above mentions that the coil affects the womb lining making it less likely to accept an egg. But of course, any schoolchild with elementary biology would tell you that eggs aren't implanted in the womb. By the time it reaches the womb, the egg will have been fertilised (fertilisation happens in the fallopian tube) and so it is not an egg that is prevented from being accepted, but an embryo. If you are on the side of the millions of doctors, lawyers, philosophers, church people and so on who believe that life begins at conception (hence all the controversy over human embryo research), then the IUD prevents a living embryo from implanting, and is, therefore, by definition, abortive.
___________________________________________________________________
Above is true but if one is a catholic then ALL forms of contraception could and are put in the same category as above - They are contralife and by extension contra God.
Another Correction:
The above is true exept for two methods: refraining from sex at periods of greatest chance of pregnancy (ie, during ovulation and a few days afterwards) - sometimes called the 'rhythm' method, and the 'Billings method' - by examining cervical mucus to determine the time of greatest chance of pregnancy and refraining from sex at that time. I am not a Catholic, and therefore have no axe to grind on this, but I think that it is vital that the asker of the question has the correct facts without bias either way.