Uterine resting tonus and maternal hypertension during late pregnancy--existence of intrauterine capacity.

The uterine resting tonus was measured with an open-end catheter beginning from the 24th gestational week through the onset of labor in order to investigate changes in the intrauterine environment from the physical aspect. The measurements were performed in 183 primiparas for a total of 272 times and 102 multiparas for a total of 120 times. The uterine resting tonus was almost constant from the 2nd trimester through the 3rd trimester, but it tended to increase gradually beginning 2-3 weeks prior to the onset of delivery. This tendency was more pronounced in primiparas, and their mean tonus was about 5mmHg higher than that of multiparas throughout the pregnancy. In 55 subjects in whom both the resting tonus and blood pressure were simultaneously measured at least twice during pregnancy, the mean maternal blood pressure increased as the resting tonus increased. The above findings revealed a positive correlation between the resting tonus and the maternal blood pressure and suggested the presence of a critical point in terms of the intrauterine capacity, which derives from the properties of the uterine muscle and the intrauterine contents, consisting mainly of the fetus. The increase in the resting tonus is surmised to be related to this critical point.