Head-to-cervix force: an important physiological variable in labour. 1. The temporal relation between head-to-cervix force and intrauterine pressure during labour

Abstract

Objective: To investigate the relation between the rise in intrauterine pressure and rise in fetal head to cervix force in normal, slow and induced labour.

Design: Prospective observational study.

Setting: The labour ward of a London teaching hospital.

Participants: Forty patients were recruited from the antenatal clinic and labour ward of a West London Hospital. Five had normal onset and progression of labour, 14 had slow progression of labour and 21 had induced onset of labour.

Method: Intrauterine pressure and head-to-cervix force was measured simultaneously using an intrauterine pressure catheter and a specially designed four sensor head-to-cervix force probe.

Results: For each contraction of each labour, scattergrams of force by pressure were plotted. Three patterns were observed. When the rise in pressure preceded the rise in force, a positive 'loop' was generated. When the rise in pressure and force occurred simultaneously a linear pattern was generated (a neutral 'loop'). When the rise in pressure lags the rise in force, a negative 'loop' was generated. In normally progressive labour the distribution of loops was 29.1%, 22.6% and 48.3%, respectively, in slow labour the distribution was 26.1%, 14.1% and 59.8% and in induced labour the distribution was 33.8%, 14.4% and 51.8%. These distributions were not statistically different. However, a higher proportion of negative loops was observed in labours augmented with oxytocin compared to those receiving no oxytocin (MW-U = 87, P = 0.036). No differences were observed comparing parity, use of PGE2, epidural analgesia, or mode of delivery. Contraction frequency (number/10 minutes) was inversely correlated to the percentage of negative loops (rs = -0.34, P = 0.033) and positively correlated with percentage of positive loops (rs = 0.36, P = 0.027).

Conclusions: This is the first report of the temporal relation between intrauterine pressure and head-to-cervix force in labour. The most common pattern is that the rise in pressure lags the rise in force, suggesting that a seal has to be created between the fetal head and cervix before a rise in pressure can occur. When oxytocin is given in labour, a higher proportion of loops are negative indicating that there is poor application of the fetal head and cervix in a greater proportion of contractions.