Approximately one-third of all human twins are identical (one-egg) and two-thirds are fraternal (two-egg) twins. Identical twins are necessarily the same sex, with male and female identical twins having equal probability. Among fraternal twins, approximately one-fourth are both female, one-fourth are both male and half are one male and one female. Finally, among all U.S. births, 1 in 80 is a twin birth. Find the probability that an U.S. birth results in identical twins that are female.