We thank Drs Ferrazzi and Paganelli for their interest in our recent commentaries on the controversial American College of Obstetricians and Gynecologists/Society for Maternal-Fetal Medicine labor management guidelines. They believe that the guidelines derive from the “properly modeled” labor curves of Zhang et al. As we argued previously, these guidelines are based on flawed analyses of labor progress. Further, the safety and effectiveness of the recommended limits of “normal” are unsupported by any evidence.

In their analysis of cervical dilation, Ferrazzi et al, like other investigators who have used interval-based approaches to curve fitting, ignore the indisputable fact that their results are inconsistent with direct clinical and experimental assessment of serial measurements of dilation and descent in individual women. No amount of retrospective mathematical manipulation of cross-sectional data will rationalize that discrepancy. The blithe indifference of investigators and policy-makers to this inconsistency is mystifying.

The study of Ferrazzi et al suffers from the same flaws that we identified elsewhere related to selection biases and unadjusted confounders. Thus, their inability to predict subsequent dilation based on clinical assessments is hardly surprising, given the nature of their curves and the way they framed the question. Other data suggest convincingly that with at least 2 assessments of dilation in the active phase, the subsequent path of dilation can be predicted reliably, which allows a benchmark for judging the normality of further progress. It is interesting, nevertheless, that the dilation curve of Ferrazzi et al, while speciously exponential like that of Zhang et al, differs from the latter in having an active-phase rate of dilation quite similar to that of Friedman.

We concur that more basic science and clinical focus on factors that influence labor progress is needed. The (Friedman) labor curves derive from empiric observation. From them, we know that certain abnormal labor patterns are associated with an increased need for cesarean delivery and with morbidity in offspring. Causation can only be inferred from such information.

Many causes of abnormal labor patterns probably act through the common mechanism of insufficient or disordered uterine contractility. Influences on contractile efficiency include infectious agents, circulating hormones and metabolites, immunologic and genetic factors, uterine oxygenation, and even biomechanical factors that are related to fetopelvic relationships. These (and many others) require study. Of course, to obtain the most meaningful results from such studies, an agreed on framework for what constitutes normal progress will be necessary. We are less sanguine than our Italian colleagues about whether the new guidelines they extol provide that framework.