Overall risk of pregnancy following a single act of unprotected intercourse on any day in the menstrual cycle is 2-4%. The risk from a single act is highest
(20-30%) in the days before and just after ovulation. Pregnancy may be prevented before implantation by disrupting the normal hormonal arrangements; the mode of action is probably by delaying or preventing ovulation or by preventing implantation of the fertilised ovum.
Postcoital contraception may be successful up to 72 hours after a single act of unprotected intercourse. A common technique is to take one tablet of levonoegestrel 750 micrograms (Levonelle 2) as soon as possible after intercourse and then a second tablet 12 hours (but not more than 16 hours) later.18 With this regimen, vomiting is rare but if it occurs within 3 hours of either dose, another table should be taken immediately. It is available 'over the counter', i.e. off prescription, in the UK from trained pharmacists.
If pregnancy is present the treatment will not cause abortion, and evidence suggests that it does not harm a fetus. The procedure is not licenced for use more than once in a cycle.
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