Factors Concerned in the Duration of Pregnancy

In the attempt to account for the emptying of the uterus at a definite time, various hypotheses have been formulated upon the basis of one or more of the changes in the fetal or maternal organism which have been noted at the time of termination of pregnancy. The viewpoint of the obstetrician, whose experience involves chiefly tlhe phenomena of labor, has been the basis of numerous theories, the multiplicity of which bears evidence of the uncertaintly attending them. A clue to the nature of parturition is found by experiment. Empty-ing of the uterus is under hormonal control; the time of onset may be hastened or postponed as desired, living offspring being born in either case. In experiments in which a new ovulation is superimposed upon pregnancy, a definite rhythm is still evident in the duration of gesta-tion; the amount of lengt!hening or shortening of pregnancy is not irregular, but is fixed according to the length of the ovulation cycle; the normal mechanism of labor seems to be in operation except, at an earlier or later time than is usual. In order to determine the relation of parturition to preceding stages of pregnancy, the method of the embryologist rather than that of t.he obstetrician must be followed. The common method of approach to a problem of embryology, whether a structure or a function is involved, is to reconstruct various stages in the development of the organ or pro-cess. The final form is viewed from the standpoint of its development. Frorn the embryological standpoint, parturition represellts merelv tlhe terminal phase of a long series of events constituting embryonic 1ifeU. The brief duration of the period of emptying of the uterus (a few minutes in rodents, a few hours in the human), in contrast to the long period of retention of the fetus, has been a major obstacle, making it difficult, to detect, tfhe factors which control the process. The question as to how far back in embryonic life one must seek for the origin of the birt!h mechanism involves investigation of the role of various endocrine organs at different stages of pregnancy. The minimum requirement of en-