[New data on the hormonal requirement of the pregnant rabbit: partial pregnancies and fetal abnormalities after treatment with a hormonal antagonist given at a sub-abortive dosage].

Evolution of pregnancy in rabbits depends upon ovarian progesterone. A deficiency in ovarian hormones was observed a long time ago to result either in abortion or in "partial pregnancy" or even in fetal anomalies. Administration to pregnant rabbits, from day 11 on, of a hormonal antagonist known for its anti-progesterone activity, RU486, at a sub-abortive dosage, reproduces the effects of ovarian hormone deficiencies.