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

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.