[Changes of intrauterine pressure in progesterone-treated ovariectomized rats during the second half of pregnancy]

Pregnant rats were ovariectomized on day 14 of pregnancy (sperm = day 1) and daily injected with 4 mg progesterone. The intrauterine pressure and volume of the conceptus with placenta of these rats and intact normal pregnant rats were measured every morning between day 14 and 20 of pregnancy. The average volume of the conceptus in both groups increased at an almost constant rate until day 18 of pregnancy and rapidly between day 19 and 20 of pregnancy. The intrauterine pressure of ovariectomized progesterone-treated rats increased in parallel with the volume of the conceptus until day 19 of pregnancy, suggesting a deficiency in plasticity of the uterine wall. The pressure, however, decreased significantly between day 19 and 20 of pregnancy in spite of the rapid increase of the conceptus volume. In the control rats, the intrauterine pressure gradually decreased during the second half of pregnancy, whereas the conceptus volume increased. The results suggest that estrogen secreted from the ovaries during the second half of pregnancy prevents the rise of the intrauterine pressure by improving the plasticity of the uterus. Furthermore, it could be considered that the decrease of the intrauterine pressure between day 19 and 20 of pregnancy in ovariectomized progesterone-treated rats may be caused by the estrogenic action of androgens secreted from the placenta in late pregnancy.