A woman does not become pregnant immediately after having sex.

The process of becoming pregnant can take as long as a week. According to Princeton University, medical science defines the beginning of pregnancy as the successful implantation of a fertilized egg in the lining of the uterus.

EBC pills can interrupt the process at several points after a woman has sex but before implantation occurs, thereby preventing her from becoming pregnant.

A woman's eggs are stored in her ovaries, and each month an egg is released from one of them. This process is called ovulation.

Once the egg is released, it travels down the fallopian tube over a period of about 7 days.

In order for pregnancy to begin, the egg must first be fertilized by sperm, and then it must successfully implant in the lining of the uterus. Sperm can fertilize the egg at any time as it travels from the ovary to the uterus. Sperm can also wait several days in the fallopian tubes for an egg to be released.

This diagram illustrates the process of fertilization.

When a man and a woman have sex, the man ejaculates his sperm into the woman's vagina. The sperm then swim up through the vagina and uterus and into the fallopian tubes (see top diagram) looking for an egg to fertilize. If an egg is available, one sperm eventually penetrates its outer membrane. The nucleus of the sperm and egg then fuse together. This fertilized egg is now called a zygote, and its cells begin to divide.

Even though the egg is fertilized, it must successfully implant in the uterus for pregancy to begin. Implantation is not always successful, even under normal circumstances when a woman is not taking regular or emergency birth control pills. If implantation does not occur, the woman simply passes the fertilized egg during menstruation.

EBC may work by interrupting the pregnancy process in any of the these ways:

1. Delaying or inhibiting ovulation

In this case, EBC prevents any egg from being released from the ovaries. With no egg for sperm to fertilize, a woman cannot become pregnant. The most current research suggests that this is usually the way that EBC pills prevent pregnancy. In fact, this is the only way in which EBC pills have been proven to work. The other ways described below have not been studied enough to be universally accepted by the medical community, and therefore remain theoretical.

2. Inhibiting tubal transport of the egg or sperm

In this case, EBC prevents the sperm and/or egg from traveling through the fallopian tubes. This is thought to occur by changing the consistency of the mucus in the fallopian tubes. Sperm travel by swimming, and eggs are propelled by microscopic hairs lining the fallopian tubes called cilia. If the sperm or egg can't travel through the tubes, they may never meet, or if they do the fertilized egg may not reach the uterus and implant.

3. Preventing fertilization

In this case, EBC may create chemical changes that prevent sperm from fertilizing an available egg even if they do meet.

4. Inhibiting implantation of a fertilized egg

In this case, EBC may prevent a fertilized egg from successfully implanting in the lining of the uterus. Menstruation is the normal process of shedding the lining of the uterus. When a woman becomes pregnant she does not menstruate, and so the lining of the uterus remains to support the developing embryo. EBC pills, the copper-T IUD, regular birth control pills, and many other drugs such as certain prescription allergy medications may all prevent pregnancy by causing the lining of the uterus to be shed even if a fertilized egg is present. Together, these medicines are prescribed to millions of women each year.

5. Stimulating an auto-immune response

It has been suggested that IUDs may work by stimulating a woman's immune system, and that her immune system in turn attacks both sperm and fertilized eggs because they contain foreign genetic material.

Although the current research suggests that both EBC pills and regular bith control pills usually work by preventing ovulation, it is important to understand that they may not always prevent fertilization.

Some people consider conception to be the moment of fertilization and not the beginning of pregnancy, and so these people believe that, just like regular birth control pills, EBC pills may not always prevent conception when they prevent pregnancy.

Anti-choice groups have specifically attacked EBC pills with claims that they cause an abortion because they may not always prevent fertilization when they prevent pregnancy. But these groups have not attacked regular birth control pills or the many other prescription medications presribed to millions of women each year that have exactly the same effect. The specific targeting of this one very important medical technology which empowers women with an extraordinary reproductive health choice suggests that the motives of these groups' objections to EBC pills are not made entirely clear to the public.

To learn more about why anti-choice groups have specifically targeted EBC pills with these attacks, see the Why do some people say that emergency birth control is an abortifacient (something that causes an abortion) ? FAQ.