HEAD-TO-CERVIX FORCE - AN IMPORTANT PHYSIOLOGICAL VARIABLE IN LABOR .2. PEAK ACTIVE FORCE, PEAK ACTIVE PRESSURE AND MODE OF DELIVERY

Objective To assess the relation between peak active (above baseline) head-to-cervix force (paHCF) and peak active (above baseline) intraute rine pressure (paIUP) in labour, and to compare the relation between l abours progressing well and ending in vaginal delivery and those labou rs progressing slowly and ending in caesarean section. Design Prospect ive observational study. Setting The labour ward of a London teaching hospital. Participants Forty women in labour who agreed to have an exp erimental head-to-cervix force probe and an intrauterine pressure cath eter inserted. Results The relation was linear, with a correlation coe fficient which ranged from 0.012 to 0.885 (mean value 0.438). The clos eness of the relation did not correlate with the rate of cervical dila tation (r = 0.0192, 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, signifi cantly steeper than in women who progressed slowly and required delive ry by caesarean section (mean slope = 0.45, t = 2.31, P = 0.02). Mean paIUPs were significantly higher in women progressing well and achievi ng a vaginal delivery than in those progressing slowly and requiring c aesarean section (45.3 mmHg SD 7.5 vs 38.4 mmHg SD 11.4, t = 2.31, P = 0.02), but the overlap between the two groups was considerable. Howev er, in keeping with their steeper paHCF/paIUP slope, women progressing well to a vaginal delivery had substantially higher head-to-cervix fo rces (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 w as much less overlap between paHCF than paIUP. This resulted in paHCF being a much better discriminating variable than paIUP for mode of del ivery. Conclusions The relation between paHCF and paIUP is linear, but there is a wide variation in the degree of correlation between one wo man and another. Women with a steep slope of paHCF relative to paIUP a re more likely to achieve a high mean paHCF, progress rapidly in labou r and achieve a vaginal delivery than women with a flap slope, althoug h 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-cer vix force is sensitive to factors determining mode of delivery which a re not reflected in either the level of uterine activity as measured b y intrauterine pressure, or cervical compliance as measured by cervica l dilatation rates.