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

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.