There are two basic ways that the drug could prevent pregnancy. It could prevent ovulation, therefore preventing an egg from being available to meet sperm. No fertilized egg means no pregnancy.
The other possibility, that the drug might prevent a fertilized egg from becoming implanted in the uterine wall, is the one that has caused so much discussion. In this scenario, an egg is released and fertilized by a sperm that got through when the condom broke or was forgotten or whatever. When the woman takes the morning after pill, if this were the case, she would be preventing an already fertilized egg from continuing its life by preventing it from implanting in the womb where it would be warm and nourished.
Those who object to the use of emergency contraception argue that the egg/sperm combo is a living creature and that we are killing it by using the morning after pill. The argument from here is pretty much the same as the argument against abortion, and many of the same people can be found in the two camps. To these folks, preventing the egg from implanting is as heinous as allowing a woman to terminate a pregnancy through abortion, as both involve the 'murder' of the egg/sperm combo.
Until now we weren't really sure how the morning after pill exerted its effect. We could see that it worked, but couldn't say which of the two mechanisms was correct. New research published in the journal Contraception provides some answers.
A randomized, double-blind, placebo-controlled study was carried out investigating the method of action of Plan B, the most common and effective form of emergency contraception. They discovered that in in 82% of the Plan B treated cycles, the women did not ovulate. This rate of non-ovulation is identical to the estimated efficacy rate of Plan B. Blood tests indicate that the drug affects ovulation by supressing the surge of lutenizing hormone that normally triggers ovulation. The researchers concluded that Plan B prevents pregnancy by preventing ovulation, rather than by preventing implantation.
They suggest that the failure rate of the morning after pill is likely the result of failure to alter the surge of lutenizing hormone. This would occur if the woman took the drug too late in the fertile preovulatory period, when ovulation had already begun.
This study was small, involving only about 60 women, but very thorough. Further research, to replicate these results with larger groups, is necessary, but this offers some insight into the way that emergency contraceptives work. Perhaps more research will quiet the anti-choice voices.