Coitus interruptus
Coitus interruptus, also known as withdrawal or the pull out method, is a method of contraception in which, during sexual intercourse, the penis is removed from the vagina prior to ejaculation, primarily to avoid introducing semen into the vagina. Coitus interruptus may also more generally refer to any extraction of the penis prior to ejaculation during intercourse. This method has been widely used for at least 2,000 years and was used by an estimated 38 million couples worldwide in 1991.

# Effectiveness
Like many methods of birth control, reliable effectiveness is achieved only by correct and consistent use. Observed failure rates of withdrawal vary depending on the population being studied: studies have found actual failure rates of 15-28% per year. In comparison the pill has an actual use failure rate of 2-8%, while the diaphragm has an actual use failure rate of 10-39%.  The condom has an actual use failure rate of 10-18%.
For couples that use withdrawal correctly at every act of intercourse, the failure rate is 4% per year. In comparison the pill has a perfect-use failure rate of 0.3%, while the diaphragm has a perfect use failure rate of 6%.  The condom has a perfect-use failure rate of 2%.
The primary cause of failure of the withdrawal method is the lack of self-control of those using it. Poor timing of the withdrawal can result in semen on the vulva, which can easily migrate into the female reproductive tract.  Some medical professionals view withdrawal as an ineffective method of birth control.  In contrast, a recent study in Iran found that provinces with higher rates of withdrawal use do not have higher fertility rates, and that the contribution of withdrawal use to unintended pregnancies is not markedly different from that of other commonly used methods such as the pill or condom.
It has been suggested that the pre-ejaculate ("Cowper's fluid"), fluid emitted by the penis prior to ejaculation, contains spermatozoa (sperm cells), which can easily pass into the upper female genital tract in the presence of cervical mucus. However, several small studies have failed to find any viable sperm in the fluid.  While no large conclusive studies have been done, it is now believed the primary cause of method (correct-use) failure is the pre-ejaculate fluid picking up sperm from a previous ejaculation.  For this reason, it is recommended that users of withdrawal have the male partner urinate between ejaculations, to clear the urethra of sperm, and wash any ejaculate from objects that might come near the woman's vulva (e.g. hands and his penis).

# Advantages
The advantage of coitus interruptus is that it can be used by people who have objections to or do not have access to other forms of contraception. (Some men prefer it so they can protect their partners from possible adverse effects of contraceptives.) It has no monetary cost, requires no artificial devices, has no physical side effects, and can be practiced without a prescription or medical consultation.

# Disadvantages
The method is largely ineffective in the prevention of STDs, like HIV, since pre-ejaculate may carry viral particles or bacteria which may infect the partner if this fluid comes in contact with mucous membranes.  However, a reduction in the volume of bodily fluids exchanged during intercourse may reduce the likelihood of disease transmission due to the smaller number of pathogens present.
The method may be difficult for some couples to use. The interruption of intercourse may leave some couples sexually frustrated or unsatisfied.

# Prevalence
Worldwide, 3% of women of childbearing age rely on withdrawal as their primary method of contraception.  Regional popularity of the method varies widely, from a low of 1% on the African continent to 16% in Western Asia. (Data from surveys during the late 1990s).
In the United States, 56% of women of reproductive age have had a partner use withdrawal.  In 2002, 2.5% were using withdrawal as their primary method of contraception.