Histological and clinical features of menstruation induced by the antiprogestin mifepristone (RU486) compared to menstruation occurring spontaneously

The amount and duration of menstruation induced by a single oral dose of mifepristone (RU38486 or RU486) given in the first half of the luteal phase did not differ from spontaneous menstruation, suggesting that the haemostatic mechanisms are similar. With endometrial biopsy specimens, menstruation induced by mifepristone, was associated with a reduction of the amount of glandular secretory activity, predecidual reaction and leucocytic infiltration, implying that these processes may not be essential for menstruation. The amount of predecidual reaction at the time of induced menstruation was found to be reduced in those who subsequently had further bleeding around the expected time of menstruation for that cycle when compared to those who did not. Predecidual reaction, though not a prerequisite for endometrial bleeding, may be closely related to endometrial shedding at the time of menstruation.