Induction of preterm labor in the rat by antiprogesterone.

The validity of a new concept, predicting that preterm labor can be induced without oxytocic stimulation by a regulatory imbalance, generated by antiprogesterone (A-P)-provoked P withdrawal (Pw), has been examined in the rat model. At day 19 of pregnancy a single oral dose of the steroidal A-O isoxazol, which inhibits P synthesis, significantly reduced uterine P levels, increased uterine estradiol and prostaglandin F levels, and induced preterm labor. This precocious regulatory imbalance and preterm labor were prevented, by blocking the A-P-induced Pw with P treatment. The regulatory imbalance which triggered the onset of spontaneous labor in the control animals was similar, but it occurred at term rather than before term. The potential of this method in improving the management of medically indicated induction of preterm labor is discussed.