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Workings of emergency contraception

From Dr Diana Lowry, Epping

Professor John Guillebaud says 'hormonal emergency contraception prevents pregnancy primarily by delaying or stopping ovulation and rarely if ever by preventing implantation of a fertilised ovum' (Clinical, 22 March).

As we have no way of being certain that a woman has or hasn't ovulated prior to unprotected intercourse, surely there will be some women for whom ovulation has already occurred. How does emergency contraception work then? By inducing shedding of the endometrium, or by preventing implantation?

Professor Guillebaud responds:

When a contraception method has more than one potential mechanism of action, research to establish how it worked in each particular case is difficult. We have to remember that a large majority of attenders for emergency contraception (estimated as 92 out of every 100 sexually exposed just the once, in the first WHO comparative study of Levonelle in 1998) would not have conceived even if untreated.

So the 1 per cent failure rate among each 100 treated within 72 hours means in reality seven conceptions not occurring out of the eight truly at risk. But nobody knows in how many of those seven the success was due to: stopping ovulation; postponing ovulation; cervico-uterine fluid being sufficiently altered by the progestogen to stop the sperm reaching an egg; or preventing implantation.All that can be said is that the successes were more likely to have been because of one of the first three mechanisms (primarily the first two, in fact) than the fourth: though as Dr Lowry says, the fourth has to have operated in some instances. If it does not, the pregnancy should not be harmed. If blocking implantation is expected to be the mechanism required, insertion of a copper IUD would be more effective.

• From Dr Toni Hazell, GP and family planning clinic doctor

As a GP and family planning doctor, I read with interest the correspondence about missed pills (Letters, 5 April). I too do not use the flowchart in the latest FPA leaflet for the combined pill. The correspondence in the Journal of the Faculty of Family Planning following the change in pill rules was almost 100 per cent against the change.

It is laughable to suggest patients know whether their pill has 20 or 30 micrograms of oestrogen in it – many of my patients would not know if their pill is combined or progesterone only, let alone the dose, and a large proportion of patients do not know the name of their pill ('it's the one in the green packet.......micro-something').The new rules allow for an extended pill-free interval of more than seven days and there is plenty of evidence to suggest some women will ovulate in this time. I believe the only reason the combined pill is so effective is because of the margin for error – the new rules remove this.With the full support of my local family planning consultant I will continue to use the old leaflets when I can get hold of them and to teach the old pill rules. I would welcome any move by the Faculty of Family Planning to actually listen to its members and return to the old rules.

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