Head‐to‐cervix force: an important physiological variable in labour. 2. Peak active force, peak active pressure and mode of delivery

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Abstract
Objective To assess the relation between peak active (above baseline) head‐to‐cervix force (paHCF) and peak active (above baseline) intrauterine pressure (paIUP) in labour, and to compare the relation between labours progressing well and ending in vaginal delivery and those labours progressing slowly and ending in caesarean section.Design Prospective observational study.Setting The labour ward of a London teaching hospital.Participants Forty women in labour who agreed to have an experimental head‐to‐cervix force probe and an intrauterine pressure catheter inserted.Results The relation was linear, with a correlation coefficient which ranged from 0.012 to 0.885 (mean value 0.438). The closeness of the relation did not correlate with the rate of cervical dilatation (r = 00192, P= 0.574) or the mode of delivery (r = 0.215, P= 0.183). However, in women who progressed well to a vaginal delivery, the mean slope of the paHCF to paIUP regression line was 0.72, significantly steeper than in women who progressed slowly and required delivery by caesarean section (mean slope = 0.45, t = 2.31, P= 0.02). Mean paIUPs were significantly higher in women progressing well and achieving a vaginal delivery than in those progressing slowly and requiring caesarean section (45.3 mmHg SD 7.5 vs 38.4 mmHg SD11.4, t = 2.31, P= 0.02), but the overlap between the two groups was considerable. However, in keeping with their steeper paHCF/paIUP slope, women progressing well to a vaginal delivery had substantially higher head‐to‐cervix forces (46.4 gWt SD 11.8 vs 28.3 gWt SD 8.2, t = 5.22, P < 0.00001) than those progressing slowly and requiring caesarean section, and there was much less overlap between paHCF than paIUP. This resulted in paHCF being a much better discriminating variable than paIUP for mode of delivery.Conclusions The relation between paHCF and paIUP is linear, but there is a wide variation in the degree of correlation between one woman and another. Women with a steep slope of paHCF relative to paIUP are more likely to achieve a high mean paHCF, progress rapidly in labour and achieve a vaginal delivery than women with a flap slope, although mean paIUPs do not differ substantially between the two groups. The level of paHCF was substantially better than cervical dilatation rate at predicting mode of delivery. These results suggest that head‐to‐cervix force is sensitive to factors determining mode of delivery which are not reflected in either the level of uterine activity as measured by intrauterine pressure, or cervical compliance as measured by cervical dilatation rates.

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