Standards in fetal monitoring--practical requirements for uterine activity measurement and recording.

External monitoring of uterine contractility in human labour can record contraction frequency accurately, but cannot determine true intra-uterine pressure reliably. It is therefore suitable for use in spontaneous labour where there is a need for a marker of contractions to aid in continuous fetal heart rate pattern interpretation. Intra-uterine pressure measurement can provide quantitative information more reliably over intervals of 10-60 minutes. Although active pressure values for individual contractions can vary by up to 50% when measured simultaneously by two or more intra-uterine catheters/transducers, this variation is not systematic and cumulative measures vary by less than 5%. The measurement of baseline tone is affected by variables not related to uterine activity and should therefore be excluded when quantitative measures are made.