In general, birth control or contraception is anything that prevents a woman from becoming pregnant.The most basic and cheapest form of contraception is abstinence (not engaging in sexual intercourse at all). However, abstaining from sex can cause great stress and frustration in some, since the human sex drive can be quite powerful.A variation on abstinence is fertility awareness (also known as "natural family planning"). This method involves a couple avoiding intercourse during the times that the woman is most likely to be fertile. This method requires that the woman be very aware of her own body and her reproductive cycle; some estimate that this method has a 70%-98% rate of success, depending on the regularity of the woman's cycle and the specific monitoring technique used. Signs that a woman has ovulated (and is therefore fertile) include a sudden change in basal body temperature, changes in vaginal mucus, or a combination of the two. A woman is least likely to be fertile during her menstrual period and for a few days thereafter. Barrier methods of birth control work by preventing sperm from reaching the egg. Barrier methods work best in conjunction with spermicides (creams or jellies that contain chemicals that kill sperm and sometimes disease-causing microorganisms). Because barrier methods also prevent the exchange of body fluids to varying degrees, they are also useful in preventing the transmission of venereal diseases. Barrier methods include: condoms female condoms diaphragms cervical caps contraceptive sponges Other birth control methods work by altering a woman's hormones so as to render her infertile (male hormonal birth control is being researched). Hormonal methods include: Depo Provera birth control pills Norplant "morning after" pills Other women may use intrauterine devices (IUDs), which work mainly by preventing the fertilized egg from implanting in the wall of the uterus. Other women use abortion (surgery to remove or drugs to induce the expulsion of an embryo or fetus) as a last-ditch effort at birth control. If a couple wants no children at all, they may undergo surgery to render themselves sterile. In women, this procedure is generally a tubal ligation (cutting or blocking the Fallopian tubes, which carry the eggs from the ovaries). In males it is a vasectomy (cutting or blocking the tubes that carry sperm from the testis). These procedures can sometimes be reversed.