Suppose you are a sexually inexperienced man, perhaps a recent apostate from one of the more devoutly celibate faiths like Mormonism or extreme Christian fundamentalism. Now imagine that a young woman has brought you back to her apartment for the sexy times and you are concerned about the possibility of pregnancy from a single liason. What are the odds, scientifically speaking? The answer is, of course, that it varies depending on many factors such as your partner’s age, fertility status, and any contraception methods that might be in play.
What is the worst case scenario for someone who is strenuously hoping to avoid fatherhood? Surprisingly, the probability can run nearly to 10%, if you happen to catch a woman right at the high point of their cycle. Here is the graph, taken from a 2001 paper entitled Likelihood of conception with a single act of intercourse: providing benchmark rates for assessment of post-coital contraceptives by Wilcox et al.
Of course, the odds are very good that you will not catch them at peak fertility, and according to the same paper, the “probability of pregnancy with one completely random act of unprotected intercourse was 3.1%” if you smear out all the probabilities across the entire cycle. Still, those aren’t good odds for the average guy who doesn’t consider himself father material just yet.
The good news is that the available contraception methods are pretty good, if you use them correctly. Here is a handy infographic from a publicly available paper entitled Contraceptive failure in the United States by James Trussell:
According to this paper, condom effectiveness ranges from 82% (typical use) to 98% (perfect use) over the course of an entire year of sexual intercourse, whereas no method whatsoever proves only 15% effective. Assuming 60 sexual encounters between a given couple over the course of a year (because maths) that would mean that wearing a condom cuts the one-time probability of clinical pregnancy from 3.1% down to around 0.0336% under optimal usage (because more maths). Those aren’t bad odds, even for a sexually inexperienced ex-Mormon frightfully fearful of fatherhood. So long as you are willing to learn how to use a condom properly, you can probably “pick a pony and bet it,” as they say in the parlance of our times.
