Our study demonstrated that five simple parameters, including maternal age, height, BMI, fetal HC, and AC measured within one week before delivery, can be used to assess the risk of CD during labor in nulliparous women. These parameters have been associated with the risk of a failed vaginal delivery during labor in several published studies [13–15]. Burke et al. combined these parameters to establish the predictive model and assess the risk of unplanned cesarean section during labor in nulliparous women . They created a predictive nomogram using similar parameters and suggested that their model has potential to offer individualized counseling and provides a reasonable option for women to undergo successful vaginal delivery . However, this model was mainly targeted at white Europeans, and showed that other ethnic groups had a higher risk of emergent cesarean section during labor than the white European group. In contrast to the model of Burke et al., we use sonographic parameters measured within one week before delivery and included all nulliparous women after 37 weeks of gestation. It is suggested that the labor induction at 39 weeks reduces the rate of CD without increasing the risk of neonatal morbidity in low-risk nulliparous women . Our nomogram included sonographic fetal sizes measured within a week of delivery. Sonographic information measured within a week before delivery was expected to provide useful information in planning labor induction before the 39th week of gestation in low-risk nulliparous women.
Our nomogram yielded slightly better performance than the previous published model . In addition, the nomogram in this study incorporated older maternal age, and wider ranges of BMI, height, HC, and AC. Because maternal age is higher in Korea and advanced maternal age is an independent risk factor for emergency cesarean section, a nomogram is needed for pregnant women aged above 40 years . In addition, several studies reported that pre-pregnancy overweight and obesity are closely related to adverse obstetric outcomes including CD or obstetric procedures [18, 19].Although the WHO classification of BMI for Asian women is modified from that of Western population, obesity and dispersion in the BMI distribution of Asians have been increased [20–23].Therefore, a nomogram with further dispersion in the range of parameters may yield better performance.
The risk of cesarean section for dystocia increases with increased neonatal birth weight . The estimated fetal weight was associated with the risk of cesarean section during labor; however, we used the fetal HC and AC separately in the predictive model instead of the estimated fetal weight. Although the accuracy of ultrasound-based fetal weight estimation has improved in recent decades, the estimated fetal weight is inconsistent with actual birth weight [25, 26]. The size of fetal head or abdomen alone is strongly correlated with actual birth weight than the estimated fetal weight [15, 27]. Stirnemann et al. suggested that fetal size should be assessed using a separate biometric measure as well as the estimated fetal weight, to avoid a minimalist approach to a single value . We agree that separate biometric measurement may be more reproducible and show consistent predictive power rather than the calculated value.
Our study has some limitations. This study was a retrospective cohort study. However, we tried to include all consecutive mothers who delivered during the study period to reduce the selection bias. The obstetrician and patients were not blinded to the sonographic information obtained via fetal biometry. The estimation itself or knowledge of the fetal weight may influence the rate of cesarean section, regardless of actual birth weight . This information may have influenced the obstetrician’s decision regarding the mode of delivery and timing of labor induction. The rate of labor induction in our study population was high at 50.6%. However, the gestation age of most women in our cohort was greater than 39 weeks, and a recent study suggested that it is reasonable to induce labor after this gestational age in low-risk women .
We could not perform external validation using an independent dataset. It may be a weakness of this study. The performance of the nomogram evaluated using the validation set showed a good fit ; however, it was relatively lower than that of the training test. This model may need to be validated in different institutions or other ethnic groups.
In conclusion, we have shown that maternal age, height, BMI at delivery, and fetal HC and AC were associated with the risk of CD during labor in nulliparous women. A predictive nomogram based on these parameters might be useful for counseling an individual parturient on the risk of cesarean section and for modifying management protocols. However, a further validation for other ethnic groups and women undergoing preterm labor is required.