Since the softening of the utero-vaginal tract accompanying the advance in pregnancy roughly parallels the change of estrogen in pregnancy urine, it seems that estrogen plays an important role in the softening mechanism of the delivery tract. Up to date there are many reports concerning the relationship of estrogen in the pregnancy urine, especially of estriol, to the foetal and placental mechanisms, but there seems to be no report on estrogen contents in pregnancy urine with observations on changes of cervical canal factors in one and the same individual. For the purpose to elucidate whether or not there exists an estrogen fraction that has a close relationship with conditions of the cervical canal the author conducted fractionation and estimation of 8 fractions of urinary estrogen with lapse of time, starting from 10 gravid months, while pursuing changes in the cervical canal factors in the same individual, and compared results in the individual with normal softening of the canal and those with delayed softening. In addition, the significance of estrogen in the field of obstetrics was studied from the aspects of level of 8 estrogen fractions and clinical course in individuals administered estriol agent. The results may be briefly summarized as follows. 1. The estriol fraction in the urine at 38 gravid weeks is above 95%. 2. Judging from the changes of each fraction, estriol and 16-epi-estriol are the fractions that are closely associated with the cervical canal. 3. In the individual with cervical canal of mature type from the beginning the level of urinary estriol is relatively high but it does not form peak; the estriol level in the case with the cervical canal in the process of transformation from immature type to the mature shows a peak once and later it decreases; and in the case with cervical canal which remains as immature type the peak is small or flat. 4. When estriol agent is administered to individual with cervical canal of immature type, the urinary estriol shows somewhat larger peak, resulting in the maturation of cervical canal and delivery time is shortened.