[The concept of fecundability in the etiologic, diagnostic and therapeutic approach to infertility (authors' transl)].

The infertility of a couple is most often attributable not to sterility but rather to subfertility in either one or both of the partners. Accordingly, the study of fertility in couples must be quantitative. Fertility can be appreciated by different indices; the most common being the fecundability, which is the probability (P) of a couple conceiving in a given menstrual cycle. It is a "risk", a concept of acknowledged value in public health. The standard epidemiological methods have led to the identification of its mean value which is of the order of 25% in our countries for young couples. It varies greatly from one couple to an other and with the numerous male and female factors on which it depends. The probability of couple obtaining a pregnancy seems to be the result of a lottery which recommences at each cycle with a probability of success P: a single formula shows how this probability increases both with P and with time. Thus, only by taking into consideration simultaneously male fertility, female fertility as well as time, an essential factor that is too often forgotten, is one able to rationally approach the etiology, diagnosis and therapeutics in case of infertility.