The last decade has transformed the physician's approach to infertility. The development of IVF for its treatment has so improved our capacity to overcome this most distressing of disabilities that we now need to revise many of our working ideas on how it can come about. This is not because new causes have been discovered. Rather, an appreciation has been gained of how apparently minor disturbances can combine to cause a major disruption in the expectation of pregnancy. At the best of times-and sometimes, no doubt, at the worst of times getting pregnant is a matter of chance. The parameter that describes this chance is the monthly probability of conception, known among epidemiologists as fecundability (f) (probability of achieving pregnancy in one menstrual cycle), and is similar to "monthly fecundity" (monthly probability of a live birth in one cycle exposed to the risk of pregnancy) (1). Like every statistic in biology, fecundability has a wide distribution of values in the population (modeled in Fig. 1). Figure 1 also illustrates a group of couples with a fecundability value of zero-meaning complete infertility, or sterility, which affects perhaps 5% of couples attempting to achieve a pregnancy.