Contribution of cervical smooth muscle activity to the duration of latent and active phases of labour.

OBJECTIVE To identify the effect of cervical muscle activity as an additional factor influencing the duration of the latent and active phase of labour. DESIGN Prospective observational study. SETTING The Ljubljana Maternity Hospital. PARTICIPANTS Fifty healthy nulliparous women requiring induction of labour having relatively unripe cervices. METHODS At the onset of labour an open-ended catheter was inserted to measure intrauterine pressure during the latent and active phase of labour. Electromyographic activity of the cervix was registered by two bipolar spiral needle electrodes placed in the transverse direction on the proximal part of the cervix in the vagina. The mean intensity of distinctive peaks in an electromyographic spectrogram were calculated in the time/frequency domain. Multiple linear regression was used to find factors affecting the duration of the latent and active phase of labour. MAIN OUTCOME MEASURES Duration of the latent and active phase of labour. RESULTS Eleven independent variables explained 64% of the variance of the latent phase duration, the most important being the effacement and consistency of the cervix, and intensity of electromyographic signals. For the duration of the active phase the same variables explained 36% of the variance; the most important variables were mean duration of uterine contractions, mean maximum intensity of uterine contractions and the newborn's head circumference. CONCLUSIONS The cervical smooth muscle activity, expressed as an electromyographic signal, contributes to the duration of the latent phase but not to the duration of the active phase. However, frequent cervical contractions are not associated with a longer latent phase.