The number is not entirely pulled out of a hat. For instance, the chance of fertilization from a single sexual act at a random part of the cycle and with no contraception being used is about 5%, based on the data of Colombo, et al. (2006). But at average sexual frequency, intercourse will be occuring multiple times per cycle, and moreover these times will be sufficiently spread out over the cycle that there is a good chance that one of them will hit the fertile window in the cycle. According to Colombo, et al., there is a 6-day period in each cycle where the conception probability ranges from 11.4% to 42.9%. The likelihood that that period will be hit by a couple who has sex at an average sexual frequency (say once or twice a week) is fairly high. Moreover, data indicates that for women in committed relationships, sexual desire increases around the time of ovulation, i.e., where the probability of conception is high. I don't know if this happens for women using oral contraception. Assuming that an increase in desire correlates with a greater likelihood of sex, this may further increase the chance of conception.

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