THE EFFECT OF INJECTED OESTROGEN ON PREGNANCY IN THE RABBIT

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Abstract
SUMMARY: Rabbits were injected with oestradiol-17β for 3–6 consecutive days at different times during pregnancy. The course of pregnancy was followed thereafter, or, in acute experiments, the staircase effect and the oxytocin sensitivity were determined on the day following the last injection. Results showed that doses of oestrogen sufficient to interrupt pregnancy through an effect on the endometrial component of the uterus did not directly antagonize the influence of endogenous progesterone on the myometrium. Diminution of the progesterone block indicated by the oxytocin sensitivity of the myometrium is considered to be due to degeneration of the placenta and hence of its ability to maintain the corpus luteum. The dosage of oestrogen necessary to interrupt pregnancy was higher in late pregnancy. This is in agreement with the finding that oestrogen production increases steadily from mid-pregnancy to term.