Blood Progesterone Levels in Relation to Parturition

The hormonal mechanism that initiates parturition is still not clearly understood, although numerous theories have been put forward to explain it. The advocates of the oxytocin theory point out that it is possible to induce a seemingly normal labour in women and rabbits by oxytocin infusion (Caldeyro-Barcia & Poseiro, 1959). On the other hand, total hypophysectomy apparently does not interfere with the normal course of parturition in women (Little, Smith, Jessiman, Selenkow, van't Hoff, Eglin & Moore, 1958), and in the rabbit oxytocin injections are only effective during the last few days of pregnancy (Schofield, 1957). If oxytocin is the factor which precipitates parturition, it would be difficult to explain those cases of twin pregnancy in women with a double uterus, in which the birth of one twin may precede that of the other by several weeks (Csapo, 1959; Kennedy, 1959). Furthermore, in the pregnant mare