birth-control, contraceptionWhen the pill is taken, what exactly happens in the body?
As the other answer says, the morning after pill prevents you from becoming pregnant by preventing the ovaries from releasing an egg in the first place, so there wont be anything for the sperm to fertilize. If an egg has already been released (depending on where you are in your cycle when you take the pill), it may also interfere with the fertilization of the egg.
Morning after pills basically contain a high dose of Progestin, which is the same hormone found in lower doses in birth control pills that are meant to be taken daily. If you plan on having unprotected sex regularly, it's safer (meaning that you will be less likely to become pregnant) if you take regular birth control pills to keep a more constant supply of Progestin in your system, rather than taking a morning after pill every time you have sex. (Also, neither of these protect against STDs; they provide protection against pregnancy only).
It's also worth noting that if the egg has already been fertilized, and that fertilized egg has had time to implant itself in the woman's uterus, the morning after pill will have no effect. You can take the pill up to 72 hours after having sex, so it doesn't have to be taken "the morning after", but the sooner the better. This pill doesn't cause any form of miscarriage or abortion if you have already become pregnant, even if you are only a couple days pregnant.
Since you can get the morning after pill over the counter without a prescription (in America, at least), it might not be a bad idea to keep a dose in your medicine cabinet so you can take it immediately if the need arises, without having to go to the pharmacy. Just make sure it doesn't expire.. :)
First of all, you can't get immediatly pregnant during sex. It takes some time for the sperm cells to join with the egg.
The morning-after pill keeps the egg from releasing. This is because the pill contains the hormone progestogen. Therefore you cannot get pregnant, because there is no egg to join with the sperm.
Article on wikipedia:
The primary mechanism of action of progestogen-only emergency contraceptive pills is to prevent fertilization by inhibition of ovulation.
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