The importance of progesterone in the maintenance of pregnancy in the sheep.

ALL MY investigations into the hormonal control of myometrial function have been carried out on lower animals. My first excursions into the ovine field were made, appropriately enough, with an obstetrician, Dr Bengtsson. We felt that someone should take up the challenge for the sheep, for Short & Moore (1959) had reported that they could find no change in the level of blood progesterone at the end of pregnancy in ewes until after the delivery of the placenta. The implication was that in the sheep, a decline in the influence of progesterone is not a necessary prelude to parturition. We were unwilling to accept this and our first series of experiments (Bengtsson & Schofield, 1960) involved recording the staircase effect shown by the myometrium, both in vivo and in vitro, the staircase effect being a functional test for progesterone dominance. We were able to demonstrate a change from a negative to a positive staircase effect before parturition, a change which indicated that the myometrium becomes oestrogen-dominated before the onset of parturition, apparently regardless of systemic blood levels of progesterone. Our next experiments were designed to discover whether normal parturition could be delayed by the daily administration of progesterone to ewes from 1 week before term. The results constituted a negative answer to this question, for parturition either occurred at the normal time or, with large doses, was delayed indefinitely and the progesterone killed the foetuses (Bengtsson & Schofield, 1963). This appeared to be evidence against the importance of a decline of progesterone influence prior to parturition. For the last 4 years I have been working on ewes during the lambing season in Devon. In the 1st year