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.