We thank Dr Sun et al for their interest in our work. We agree that radiographic pelvimetry provides a definitive answer in only a small portion of patients; however, it may be useful in certain women who are considering a trial of labor after cesarean delivery, particularly in those whose previous cesarean delivery was for arrest of dilation or descent or with suspected cephalopelvic disproportion. Although our findings suggest that a woman with an anteroposterior diameter of <5th percentile should not attempt a trial of labor after cesarean delivery at term, a woman with an anteroposterior diameter of >10th percentile potentially could deliver vaginally, which would avoid repeat cesarean deliveries and their associated complications.

We also agree that x-ray pelvimetry is capable of assessing only the bony pelvis, which is necessary but not sufficient for a vaginal delivery. Other contributors to a vaginal delivery include uterine contractions, fetal size, or fetal tolerance to labor. As such, we were unable to demonstrate mid pelvis measurements above which nobody underwent cesarean delivery. The addition of a third-trimester ultrasound scan for fetal growth assessment can add to the predictive value of x-ray pelvimetry. In this population, the combination of fetal biometric measures and pelvimetry to calculate the fetal-pelvic index had a positive predictive value of cesarean delivery of 76% and a negative predictive value of 87%.

Magnetic resonance imaging (MRI) is a nonionizing alternative to x-ray pelvimetry. However, MRI is time-consuming, expensive, and more difficult to interpret than x-ray films. Many patients are unable to tolerate the procedure because of an inability to remain still for long periods of time or anxiety caused by the enclosed space. Moreover, to our knowledge, MRI pelvimetry has not been demonstrated to be more effective at the prediction of cesarean delivery than x-ray pelvimetry. Given the expense and inconvenience, MRI should be demonstrated to be equivalent or better than x-ray pelvimetry before being put into routine clinical use.