Like many methods of birth control, reliable effect is achieved only by correct and consistent use. Observed failure rates of withdrawal vary depending on the population being studied: American studies have found actual failure rates of 15–28% per year. One U.S. study, based on self-reported data from the 2006-2010 cycle of the National Survey of Family Growth, found significant differences in failure rate based on parity status. Women with 0 previous births had a 12-month failure rate of only 8.4%, which then increased to 20.4% for those with 1 prior birth and again to 27.7% for those with 2 or more. An analysis of Demographic and Health Surveys in 43 developing countries between 1990 and 2013 found a median 12-month failure rate across subregions of 13.4%, with a range of 7.8-17.1%. Individual countries within the subregions were even more varied. A large scale study of women in England and Scotland during 1968–1974 to determine the efficacy of various contraceptive methods found a failure rate of 6.7 per 100 woman-years of use. This was a “typical use” failure rate, including user failure to use the method correctly. In comparison, the combined oral contraceptive pill has an actual use failure rate of 2–8%, - see table First-Year Contraceptive Failure Rates while intrauterine devices (IUDs) have an actual use failure rate of 0.1–0.8%. Condoms have an actual use failure rate of 10–18%. However, some authors suggest that actual effectiveness of withdrawal could be similar to the effectiveness of condoms; this area needs further research. (See Comparison of birth control methods.) For couples that use<unk>  consistently and correctly at every act of intercourse, the failure rate is 4% per year. This rate is derived from an educated guess based on a modest chance of sperm in the pre-ejaculate. In comparison, the pill has a perfect-use failure rate of 0.3%, IUDs a rate of 0.1-0.6%, and internal condoms a rate of 2%. It has been suggested that the pre-ejaculate ("Cowper's fluid") emitted by the penis prior to ejaculation may contain spermatozoa (sperm cells), which would compromise the effectiveness of the method. However, several small studies have failed to find any viable sperm in the fluid. While no large conclusive studies have been done, it is believed by some that the 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 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 penis). However, recent research suggests that this might not be accurate. A contrary, yet non-generalizable study that found mixed evidence, including individual cases of a high sperm concentration, was published in March 2011. A noted limitation to these previous studies' findings is that pre-ejaculate samples were analyzed after the critical two-minute point. That is, looking for motile sperm in small amounts of pre-ejaculate via microscope after two minutes – when the sample has most likely dried – makes examination and evaluation "extremely difficult". Thus, in March 2011 a team of researchers assembled 27 male volunteers and analyzed their pre-ejaculate samples within two minutes after producing them. The researchers found that 11 of the 27 men (41%) produced pre-ejaculatory samples that contained sperm, and 10 of these samples (37%) contained a "fair amount" of motile sperm (i.e. as few as 1 million to as many as 35 million). This study therefore recommends, in order to minimize unintended pregnancy and disease transmission, the use of condoms from the first moment of genital contact. As a point of reference, a study showed that, of couples who conceived within a year of trying, only 2.5% included a male partner with a total sperm count (per ejaculate) of 23 million sperm or less. However, across a wide range of observed values, total sperm count (as with other identified semen and sperm characteristics) has weak power to predict which couples are at risk of pregnancy. Regardless, this study introduced the concept that some men may consistently have sperm in their pre-ejaculate, due to a "leakage," while others may not. Similarly, another robust study performed in 2016 found motile sperm in the pre-ejaculate of 16.7% (7/42) healthy men. What more, this study attempted to exclude contamination of sperm from ejaculate by drying the pre-ejaculate specimens to reveal a fern-like pattern, characteristics of true pre-ejaculate. All pre-ejaculate specimens were examined within an hour of production and then dried; all pre-ejaculate specimens were found to be true pre-ejaculate. It is widely believed that urinating after an ejaculation will flush the urethra of remaining sperm. However, some of the subjects in the March 2011 study who produced sperm in their pre-ejaculate did urinate (sometimes more than once) before producing their sample. Therefore, some males can release the pre-ejaculate fluid containing sperm without a previous ejaculation.
Coitus interruptus
Echinostoma
Gonyaulax
Necator americanus
Craniopagus parasiticus
Ephedra distachya
Schistosoma intercalatum
Mitragyna speciosa