Correlation between bituberous diameter and mode of delivery in a cohort of low-risk nulliparous women

Objective: The aim our study was to evaluate the association between the antepartum clinical measurement of the Bituberous Diameter (BTD) and the occurrence of unplanned obstetrical intervention (UOI) due to labor dystocia, including either operative vaginal delivery or caesarean section in a cohort of low-risk, nulliparous at term.Design: Retrospective analysis of prospectively collected data. Setting: Tertiary maternity care. Interventions: With the women lying in lithotomic the distance between two ischial tuberosities was assessed using a tape measure during the routine antenatal booking between 37 and 38 weeks of gestation. Measurements and findings: Overall, 116 patient were included, and of these 23(19.8%) were submitted to an UOI due to labor dystocia. Compared to women that had a spontaneous vaginal delivery, women submitted to an UOI had a shorter BTD (8.25 + 0.843 vs 9.60 + 1.12, p < 0.001), a higher frequency of epidural analgesia (21/23 or 91.3% vs 50/93 or 53.8%; p = 0.002) and of augmentation of labor (14/23 or 60.9% vs 19/93 or 20.4%; p < 0.001) as well as a longer first [455 (IQR 142-455 min vs 293 (IQR 142-455) min] and second stages of labor [129 (IQR 85-155) min vs 51 (IQR 27-78) min]. Multivariable logistic regression showed that the BTD (aOR 0.16, 95% CI 0.04-0.60; p = 0.007) and the length of the second stage of labor (aOR 6.83, 95% CI 2.10-22.23; p = 0.001) were independently associated with UOI. When evaluating the diagnostic accuracy of the BTD for the prediction of UOI due to labor dystocia, the BTD showed an AUC of 0.82 (95 %CI 0.73-0.91; p < 0.001) with an optimal cut-off value of 8.6 cm (78.3% (95 %CI 56.3-92.5) sensitivity, 77.4% (95 %CI 67.6-85.4) specificity, 46.2% (95% CI 30.1-62.8) PPV, 93.5% (95% CI 85.5-97.9) NPV, 3.5 (95% CI 2.3-5.4) positive LR, and 0.28 (95% CI 0.13-0.61) negative LR. A significant inverse correlation between the length of the second stage of labour and the BTD in patients that had a vaginal delivery was also demonstrated (Spearman's rho = -0.24, p = 0.01).Key conclusions: Our study suggests that antepartum clinical assessment of the BTD might be used as a reliable predictor of UOI due to labor dystocia in low-risk, nulliparous women at term gestation. Implications for practice: Antenatal identification of women at higher risk for labor dystocia might trigger some interventions during the second stage of labor, such as maternal position shifting, to increase the pelvic capacity and potentially improve outcomes or might prompt a referral of the patient to a district hospital prior to the onset of labor.