Intrauterine pressure wave-form characteristics of spontaneous first stage labor.

Intrauterine pressure wave-form parameters were measured in 827 contractions obtained from 26 patients in spontaneous labor. The coefficients of correlation between the maximal and minimal rates of pressure change and the maximal pressure amplitude were 0.78 and 0.63, respectively, and greater than or equal to 0.70 in 22/26 patients. Contractions partitioned into decile statistical groups of the pressure amplitude and both maximal and minimal rates. A linear relationship between these parameters has therefore been established. Contractions of greater amplitude tend to be longer, but the relationship between duration and amplitude is nonlinear with a limiting maximum contraction time. The duration of the midportion of the pressure wave appears invariate with respect to wave amplitude and only start-up and termination times increase with increasing amplitude. Mean values and standard deviations of the maximal amplitude (40.4 +/- 16.9mmHg). the maximal (2.4 +/- 0.9 mmHg/s) and minimal (-2.1+/- 0.9 mmHg/s)rates of pressure change, and the total duration of contractions (68.6 +/- 17.8s) were determined.