The monthly chance of conception, the technical term for which is fecundability.

Nature gives fertility a wide range, normally from about 7 to 45 percent per month for women in their early twenties, to about 3 to 25 percent per month for women in their early 30s; for women in their early 40s, it ranges from less than 1 to about 5 percent per month.

The accumulating chance, month after month, of successfully having gotten pregnant. With a monthly fertility (a monthly chance of pregnancy, or fecundability) of, say, 20 percent, there's a 20% chance of pregnancy by the end of the first month, a 20% of 80% (= 16%) chance of pregnancy in the second month, and so a cumulative 36% (20%+16%) chance by the end of the second month.

Technical speak for the monthly chance of pregnancy, or monthly fertility rate, either for an individual (measured over time) or for a population (the number of conceptions occurring in one month). For any individual with unexplained infertility and in milder cases of subfertility, chiefly determined by the duration of infertility (or, in cases of secondary infertility, in which past fecundability is being estimated, determined by time to pregnancy). Once calculated this way, an estimate of the chance of still getting pregnant naturally (as opposed to undergoing assisted conception) will chiefly be determined by the time left for conception.

American for the monthly chance of conception, or monthly fertility rate.

Infertility that is not 'absolute', or 'complete infertility' -- that is, there is a chance of pregnancy: it is not sterility (these last 3 terms are synonyms) -- but the chance of getting pregnant each month (fecundability or monthly fertility) is reduced. More or less definite causes can include oligospermia, polycystic ovary syndrome and other causes of oligomenorrhea, endometriosis, peritubal adhesions, fibroids (especially submucous fibroids) and increased age, especially of the woman. Synonymous with relative infertility.