The effect of oestrogen on uterine plasticity in late pregnant rats.

Intrauterine pressure was recorded in anaesthetized rats on Day 20 of pregnancy by the replacement of a conceptus with a balloon. Intrauterine pressure in ovariectomized pregnant rats treated with progesterone alone was markedly higher than that in intact pregnant rats, but treatment with oestradiol as well reduced values to those of controls. Oestriol and oestrone were less effective. Uterine tolerance, tested by increasing the balloon volume until the uterus ruptured, was greater than 6.5 ml in intact rats and in ovariectomized rats treated with progesterone + oestradiol, but rupture occurred at a volume of 4.3 ml when progesterone alone was used. One of the roles of oestrogen during late pregnancy in rats may be to ensure sufficient plasticity of the uterus to allow for the increase in conceptus size.