Contraceptives serve to interrupt the process of the sperm meeting the egg and implanting in the uterus, otherwise known as the events of conception. This can be achieved several ways: by preventing the release of the egg or sperm, by blocking the sperm from entering the woman's body, by blocking the sperm from traveling up through the cervix, or by preventing implantation of the early embryo in the wall of the uterus.

The concept of "effectiveness"

Contraceptives are described in terms of how effective they are. A method that is 99 percent effective allows only one woman in a hundred to get pregnant over the course of one year. If no method of birth control is used, there is a 30 percent chance of becoming pregnant each month--which translates into about 80 percent of women becoming pregnant during a year of regular intercourse. The failure rate of any given contraceptive is simply determined by the percentage of women who get pregnant in one year, despite using that type of birth control.

Differing methods of calculating the failure rate account for the varied pregnancy rates quoted for a given method of birth control. Do you include the people who say they were using a method, but who didn't use it consistently, or do you only account for those who used the method exactly according to its directions? The series of articles on birth control here will try to give you a feel for the effectiveness of each method seen both with perfect use and with more typical use.

Improving the effectiveness

When two methods are used simultaneously, their effectiveness multiplies, so that even two relatively ineffective methods can offer good protection. For instance, if a couple uses spermicidal foam (which has a 20 percent per year failure rate) with condoms (12 percent per year real-life failure rate), the risk of getting pregnant in a year is .20 multiplied by .12, or about 2 percent--a big improvement over either method alone.

Factors that contribute to the effectiveness of a method

Whether you actually use your chosen method of birth control or it stays in the bedside drawer certainly affects how well it works. Your age also influences your chances of getting pregnant using a given method of contraception. A 45-year-old has low fertility to start with, so she can use a less effective method and still expect not to conceive. Women who are in their first six months of breastfeeding are similarly "subfertile." Teenagers have high failure rates with most methods, so it's usually recommended that younger women who are sexually active use a double method of birth control, like pills and condoms.

Risks vs. benefits

Speaking of condoms, contraceptives sometimes offer other benefits besides preventing pregnancy. Many of the barrier methods also help to protect against sexually transmitted infections. Some of the hormonal methods can improve acne, treat menstrual cramps, and even decrease the chance of your developing certain cancers later in life. Each article on contraceptive options will note the noncontraceptive benefits as well as any risks.

How to choose a method

The key to choosing the right method is to know yourself and your partner and how reliable you will be with your method. You'll also want to consider the advantages and disadvantages of the method and how you will tolerate the risks and inconveniences of each choice. Think about whether you want a method that will be used only when you have sex or a pill that you have to take every day regardless of your sexual activity. Do you want something that you can buy off the shelf at the drugstore or a method that must be prescribed by a physician? How effective a method do you need? How would you feel if your method failed and you became pregnant? As you make your choice, keep in mind that using wishful thinking has an 80 percent failure rate.

The related articles on birth control provide a lot of information about each method. Your individual situation will determine which contraceptive is best for you. And don't forget to talk to your healthcare practitioner; she can give you more specific information about the risks and benefits of each for you individually to help you make a wise choice.